From: Lorenzo Stoakes Date: Fri, 9 May 2025 12:13:34 +0000 (+0100) Subject: mm: introduce new .mmap_prepare() file callback X-Git-Tag: v6.16-rc1~92^2~56 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=c84bf6dd2b83;p=thirdparty%2Flinux.git mm: introduce new .mmap_prepare() file callback Patch series "eliminate mmap() retry merge, add .mmap_prepare hook", v2. During the mmap() of a file-backed mapping, we invoke the underlying driver file's mmap() callback in order to perform driver/file system initialisation of the underlying VMA. This has been a source of issues in the past, including a significant security concern relating to unwinding of error state discovered by Jann Horn, as fixed in commit 5de195060b2e ("mm: resolve faulty mmap_region() error path behaviour") which performed the recent, significant, rework of mmap() as a whole. However, we have had a fly in the ointment remain - drivers have a great deal of freedom in the .mmap() hook to manipulate VMA state (as well as page table state). This can be problematic, as we can no longer reason sensibly about VMA state once the call is complete (the ability to do - anything - here does rather interfere with that). In addition, callers may choose to do odd or unusual things which might interfere with subsequent steps in the mmap() process, and it may do so and then raise an error, requiring very careful unwinding of state about which we can make no assumptions. Rather than providing such an open-ended interface, this series provides an alternative, far more restrictive one - we expose a whitelist of fields which can be adjusted by the driver, along with immutable state upon which the driver can make such decisions: struct vm_area_desc { /* Immutable state. */ struct mm_struct *mm; unsigned long start; unsigned long end; /* Mutable fields. Populated with initial state. */ pgoff_t pgoff; struct file *file; vm_flags_t vm_flags; pgprot_t page_prot; /* Write-only fields. */ const struct vm_operations_struct *vm_ops; void *private_data; }; The mmap logic then updates the state used to either merge with a VMA or establish a new VMA based upon this logic. This is achieved via new file hook .mmap_prepare(), which is, importantly, invoked very early on in the mmap() process. If an error arises, we can very simply abort the operation with very little unwinding of state required. The existing logic contains another, related, peccadillo - since the .mmap() callback might do anything, it may also cause a previously unmergeable VMA to become mergeable with adjacent VMAs. Right now the logic will retry a merge like this only if the driver changes VMA flags, and changes them in such a way that a merge might succeed (that is, the flags are not 'special', that is do not contain any of the flags specified in VM_SPECIAL). This has also been the source of a great deal of pain - it's hard to reason about an .mmap() callback that might do - anything - but it's also hard to reason about setting up a VMA and writing to the maple tree, only to do it again utilising a great deal of shared state. Since .mmap_prepare() sets fields before the first merge is even attempted, the use of this callback obviates the need for this retry merge logic. A driver may only specify .mmap_prepare() or the deprecated .mmap() callback. In future we may add futher callbacks beyond .mmap_prepare() to faciliate all use cass as we convert drivers. In researching this change, I examined every .mmap() callback, and discovered only a very few that set VMA state in such a way that a. the VMA flags changed and b. this would be mergeable. In the majority of cases, it turns out that drivers are mapping kernel memory and thus ultimately set VM_PFNMAP, VM_MIXEDMAP, or other unmergeable VM_SPECIAL flags. Of those that remain I identified a number of cases which are only applicable in DAX, setting the VM_HUGEPAGE flag: * dax_mmap() * erofs_file_mmap() * ext4_file_mmap() * xfs_file_mmap() For this remerge to not occur and to impact users, each of these cases would require a user to mmap() files using DAX, in parts, immediately adjacent to one another. This is a very unlikely usecase and so it does not appear to be worthwhile to adjust this functionality accordingly. We can, however, very quickly do so if needed by simply adding an .mmap_prepare() callback to these as required. There are two further non-DAX cases I idenitfied: * orangefs_file_mmap() - Clears VM_RAND_READ if set, replacing with VM_SEQ_READ. * usb_stream_hwdep_mmap() - Sets VM_DONTDUMP. Both of these cases again seem very unlikely to be mmap()'d immediately adjacent to one another in a fashion that would result in a merge. Finally, we are left with a viable case: * secretmem_mmap() - Set VM_LOCKED, VM_DONTDUMP. This is viable enough that the mm selftests trigger the logic as a matter of course. Therefore, this series replace the .secretmem_mmap() hook with .secret_mmap_prepare(). This patch (of 3): Provide a means by which drivers can specify which fields of those permitted to be changed should be altered to prior to mmap()'ing a range (which may either result from a merge or from mapping an entirely new VMA). Doing so is substantially safer than the existing .mmap() calback which provides unrestricted access to the part-constructed VMA and permits drivers and file systems to do 'creative' things which makes it hard to reason about the state of the VMA after the function returns. The existing .mmap() callback's freedom has caused a great deal of issues, especially in error handling, as unwinding the mmap() state has proven to be non-trivial and caused significant issues in the past, for instance those addressed in commit 5de195060b2e ("mm: resolve faulty mmap_region() error path behaviour"). It also necessitates a second attempt at merge once the .mmap() callback has completed, which has caused issues in the past, is awkward, adds overhead and is difficult to reason about. The .mmap_prepare() callback eliminates this requirement, as we can update fields prior to even attempting the first merge. It is safer, as we heavily restrict what can actually be modified, and being invoked very early in the mmap() process, error handling can be performed safely with very little unwinding of state required. The .mmap_prepare() and deprecated .mmap() callbacks are mutually exclusive, so we permit only one to be invoked at a time. Update vma userland test stubs to account for changes. Link: https://lkml.kernel.org/r/cover.1746792520.git.lorenzo.stoakes@oracle.com Link: https://lkml.kernel.org/r/adb36a7c4affd7393b2fc4b54cc5cfe211e41f71.1746792520.git.lorenzo.stoakes@oracle.com Signed-off-by: Lorenzo Stoakes Reviewed-by: Vlastimil Babka Cc: Al Viro Cc: Christian Brauner Cc: David Hildenbrand Cc: Jan Kara Cc: Jann Horn Cc: Liam Howlett Cc: Matthew Wilcox (Oracle) Cc: Michal Hocko Cc: Mike Rapoport Cc: Suren Baghdasaryan Signed-off-by: Andrew Morton --- diff --git a/include/linux/fs.h b/include/linux/fs.h index 016b0fe1536e3..e2721a1ff13da 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2169,6 +2169,7 @@ struct file_operations { int (*uring_cmd)(struct io_uring_cmd *ioucmd, unsigned int issue_flags); int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *, unsigned int poll_flags); + int (*mmap_prepare)(struct vm_area_desc *); } __randomize_layout; /* Supports async buffered reads */ @@ -2238,11 +2239,35 @@ struct inode_operations { struct offset_ctx *(*get_offset_ctx)(struct inode *inode); } ____cacheline_aligned; +/* Did the driver provide valid mmap hook configuration? */ +static inline bool file_has_valid_mmap_hooks(struct file *file) +{ + bool has_mmap = file->f_op->mmap; + bool has_mmap_prepare = file->f_op->mmap_prepare; + + /* Hooks are mutually exclusive. */ + if (WARN_ON_ONCE(has_mmap && has_mmap_prepare)) + return false; + if (WARN_ON_ONCE(!has_mmap && !has_mmap_prepare)) + return false; + + return true; +} + static inline int call_mmap(struct file *file, struct vm_area_struct *vma) { + if (WARN_ON_ONCE(file->f_op->mmap_prepare)) + return -EINVAL; + return file->f_op->mmap(file, vma); } +static inline int __call_mmap_prepare(struct file *file, + struct vm_area_desc *desc) +{ + return file->f_op->mmap_prepare(desc); +} + extern ssize_t vfs_read(struct file *, char __user *, size_t, loff_t *); extern ssize_t vfs_write(struct file *, const char __user *, size_t, loff_t *); extern ssize_t vfs_copy_file_range(struct file *, loff_t , struct file *, diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index e76bade9ebb12..15808cad2bc1a 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -763,6 +763,30 @@ struct vma_numab_state { int prev_scan_seq; }; +/* + * Describes a VMA that is about to be mmap()'ed. Drivers may choose to + * manipulate mutable fields which will cause those fields to be updated in the + * resultant VMA. + * + * Helper functions are not required for manipulating any field. + */ +struct vm_area_desc { + /* Immutable state. */ + struct mm_struct *mm; + unsigned long start; + unsigned long end; + + /* Mutable fields. Populated with initial state. */ + pgoff_t pgoff; + struct file *file; + vm_flags_t vm_flags; + pgprot_t page_prot; + + /* Write-only fields. */ + const struct vm_operations_struct *vm_ops; + void *private_data; +}; + /* * This struct describes a virtual memory area. There is one of these * per VM-area/task. A VM area is any part of the process virtual memory diff --git a/mm/memory.c b/mm/memory.c index 68c1d962d0ad2..99af83434e7c5 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -527,10 +527,11 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr, dump_page(page, "bad pte"); pr_alert("addr:%px vm_flags:%08lx anon_vma:%px mapping:%px index:%lx\n", (void *)addr, vma->vm_flags, vma->anon_vma, mapping, index); - pr_alert("file:%pD fault:%ps mmap:%ps read_folio:%ps\n", + pr_alert("file:%pD fault:%ps mmap:%ps mmap_prepare: %ps read_folio:%ps\n", vma->vm_file, vma->vm_ops ? vma->vm_ops->fault : NULL, vma->vm_file ? vma->vm_file->f_op->mmap : NULL, + vma->vm_file ? vma->vm_file->f_op->mmap_prepare : NULL, mapping ? mapping->a_ops->read_folio : NULL); dump_stack(); add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE); diff --git a/mm/mmap.c b/mm/mmap.c index 81dd962a1cfca..50f902c08341a 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -475,7 +475,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr, vm_flags &= ~VM_MAYEXEC; } - if (!file->f_op->mmap) + if (!file_has_valid_mmap_hooks(file)) return -ENODEV; if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP)) return -EINVAL; diff --git a/mm/vma.c b/mm/vma.c index 1f2634b295684..3f32e04bb6cc3 100644 --- a/mm/vma.c +++ b/mm/vma.c @@ -17,6 +17,11 @@ struct mmap_state { unsigned long pglen; unsigned long flags; struct file *file; + pgprot_t page_prot; + + /* User-defined fields, perhaps updated by .mmap_prepare(). */ + const struct vm_operations_struct *vm_ops; + void *vm_private_data; unsigned long charged; bool retry_merge; @@ -40,6 +45,7 @@ struct mmap_state { .pglen = PHYS_PFN(len_), \ .flags = flags_, \ .file = file_, \ + .page_prot = vm_get_page_prot(flags_), \ } #define VMG_MMAP_STATE(name, map_, vma_) \ @@ -2385,6 +2391,10 @@ static int __mmap_new_file_vma(struct mmap_state *map, int error; vma->vm_file = get_file(map->file); + + if (!map->file->f_op->mmap) + return 0; + error = mmap_file(vma->vm_file, vma); if (error) { fput(vma->vm_file); @@ -2441,7 +2451,7 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap) vma_iter_config(vmi, map->addr, map->end); vma_set_range(vma, map->addr, map->end, map->pgoff); vm_flags_init(vma, map->flags); - vma->vm_page_prot = vm_get_page_prot(map->flags); + vma->vm_page_prot = map->page_prot; if (vma_iter_prealloc(vmi, vma)) { error = -ENOMEM; @@ -2528,6 +2538,56 @@ static void __mmap_complete(struct mmap_state *map, struct vm_area_struct *vma) vma_set_page_prot(vma); } +/* + * Invoke the f_op->mmap_prepare() callback for a file-backed mapping that + * specifies it. + * + * This is called prior to any merge attempt, and updates whitelisted fields + * that are permitted to be updated by the caller. + * + * All but user-defined fields will be pre-populated with original values. + * + * Returns 0 on success, or an error code otherwise. + */ +static int call_mmap_prepare(struct mmap_state *map) +{ + int err; + struct vm_area_desc desc = { + .mm = map->mm, + .start = map->addr, + .end = map->end, + + .pgoff = map->pgoff, + .file = map->file, + .vm_flags = map->flags, + .page_prot = map->page_prot, + }; + + /* Invoke the hook. */ + err = __call_mmap_prepare(map->file, &desc); + if (err) + return err; + + /* Update fields permitted to be changed. */ + map->pgoff = desc.pgoff; + map->file = desc.file; + map->flags = desc.vm_flags; + map->page_prot = desc.page_prot; + /* User-defined fields. */ + map->vm_ops = desc.vm_ops; + map->vm_private_data = desc.private_data; + + return 0; +} + +static void set_vma_user_defined_fields(struct vm_area_struct *vma, + struct mmap_state *map) +{ + if (map->vm_ops) + vma->vm_ops = map->vm_ops; + vma->vm_private_data = map->vm_private_data; +} + static unsigned long __mmap_region(struct file *file, unsigned long addr, unsigned long len, vm_flags_t vm_flags, unsigned long pgoff, struct list_head *uf) @@ -2535,10 +2595,13 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr, struct mm_struct *mm = current->mm; struct vm_area_struct *vma = NULL; int error; + bool have_mmap_prepare = file && file->f_op->mmap_prepare; VMA_ITERATOR(vmi, mm, addr); MMAP_STATE(map, mm, &vmi, addr, len, pgoff, vm_flags, file); error = __mmap_prepare(&map, uf); + if (!error && have_mmap_prepare) + error = call_mmap_prepare(&map); if (error) goto abort_munmap; @@ -2556,6 +2619,9 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr, goto unacct_error; } + if (have_mmap_prepare) + set_vma_user_defined_fields(vma, &map); + /* If flags changed, we might be able to merge, so try again. */ if (map.retry_merge) { struct vm_area_struct *merged; diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h index 198abe66de5a9..f6e45e62da3a6 100644 --- a/tools/testing/vma/vma_internal.h +++ b/tools/testing/vma/vma_internal.h @@ -253,8 +253,40 @@ struct mm_struct { unsigned long flags; /* Must use atomic bitops to access */ }; +struct vm_area_struct; + +/* + * Describes a VMA that is about to be mmap()'ed. Drivers may choose to + * manipulate mutable fields which will cause those fields to be updated in the + * resultant VMA. + * + * Helper functions are not required for manipulating any field. + */ +struct vm_area_desc { + /* Immutable state. */ + struct mm_struct *mm; + unsigned long start; + unsigned long end; + + /* Mutable fields. Populated with initial state. */ + pgoff_t pgoff; + struct file *file; + vm_flags_t vm_flags; + pgprot_t page_prot; + + /* Write-only fields. */ + const struct vm_operations_struct *vm_ops; + void *private_data; +}; + +struct file_operations { + int (*mmap)(struct file *, struct vm_area_struct *); + int (*mmap_prepare)(struct vm_area_desc *); +}; + struct file { struct address_space *f_mapping; + const struct file_operations *f_op; }; #define VMA_LOCK_OFFSET 0x40000000 @@ -1125,11 +1157,6 @@ static inline void vm_flags_clear(struct vm_area_struct *vma, vma->__vm_flags &= ~flags; } -static inline int call_mmap(struct file *, struct vm_area_struct *) -{ - return 0; -} - static inline int shmem_zero_setup(struct vm_area_struct *) { return 0; @@ -1405,4 +1432,33 @@ static inline void free_anon_vma_name(struct vm_area_struct *vma) (void)vma; } +/* Did the driver provide valid mmap hook configuration? */ +static inline bool file_has_valid_mmap_hooks(struct file *file) +{ + bool has_mmap = file->f_op->mmap; + bool has_mmap_prepare = file->f_op->mmap_prepare; + + /* Hooks are mutually exclusive. */ + if (WARN_ON_ONCE(has_mmap && has_mmap_prepare)) + return false; + if (WARN_ON_ONCE(!has_mmap && !has_mmap_prepare)) + return false; + + return true; +} + +static inline int call_mmap(struct file *file, struct vm_area_struct *vma) +{ + if (WARN_ON_ONCE(file->f_op->mmap_prepare)) + return -EINVAL; + + return file->f_op->mmap(file, vma); +} + +static inline int __call_mmap_prepare(struct file *file, + struct vm_area_desc *desc) +{ + return file->f_op->mmap_prepare(desc); +} + #endif /* __MM_VMA_INTERNAL_H */