From f0b79944e6f41b1a242b128fb2702378eb48fbd2 Mon Sep 17 00:00:00 2001 From: Jordan Rome Date: Thu, 13 Feb 2025 07:21:23 -0800 Subject: [PATCH] mm: Add copy_remote_vm_str() for readng C strings from remote VM Similar to `access_process_vm()` but specific to strings. Also chunks reads by page and utilizes `strscpy()` for handling null termination. The primary motivation for this change is to copy strings from a non-current task/process in BPF. There is already a helper `bpf_copy_from_user_task()`, which uses `access_process_vm()` but one to handle strings would be very helpful. Signed-off-by: Jordan Rome Signed-off-by: Andrii Nakryiko Reviewed-by: Shakeel Butt Link: https://lore.kernel.org/bpf/20250213152125.1837400-1-linux@jordanrome.com --- include/linux/mm.h | 5 ++ mm/memory.c | 118 +++++++++++++++++++++++++++++++++++++++++++++ mm/nommu.c | 79 ++++++++++++++++++++++++++++++ 3 files changed, 202 insertions(+) diff --git a/include/linux/mm.h b/include/linux/mm.h index 7b1068ddcbb70..5409c7b505094 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2486,6 +2486,11 @@ extern int access_process_vm(struct task_struct *tsk, unsigned long addr, extern int access_remote_vm(struct mm_struct *mm, unsigned long addr, void *buf, int len, unsigned int gup_flags); +#ifdef CONFIG_BPF_SYSCALL +extern int copy_remote_vm_str(struct task_struct *tsk, unsigned long addr, + void *buf, int len, unsigned int gup_flags); +#endif + long get_user_pages_remote(struct mm_struct *mm, unsigned long start, unsigned long nr_pages, unsigned int gup_flags, struct page **pages, diff --git a/mm/memory.c b/mm/memory.c index 539c0f7c6d545..37dec50d73bc7 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -6803,6 +6803,124 @@ int access_process_vm(struct task_struct *tsk, unsigned long addr, } EXPORT_SYMBOL_GPL(access_process_vm); +#ifdef CONFIG_BPF_SYSCALL +/* + * Copy a string from another process's address space as given in mm. + * If there is any error return -EFAULT. + */ +static int __copy_remote_vm_str(struct mm_struct *mm, unsigned long addr, + void *buf, int len, unsigned int gup_flags) +{ + void *old_buf = buf; + int err = 0; + + *(char *)buf = '\0'; + + if (mmap_read_lock_killable(mm)) + return -EFAULT; + + addr = untagged_addr_remote(mm, addr); + + /* Avoid triggering the temporary warning in __get_user_pages */ + if (!vma_lookup(mm, addr)) { + err = -EFAULT; + goto out; + } + + while (len) { + int bytes, offset, retval; + void *maddr; + struct page *page; + struct vm_area_struct *vma = NULL; + + page = get_user_page_vma_remote(mm, addr, gup_flags, &vma); + if (IS_ERR(page)) { + /* + * Treat as a total failure for now until we decide how + * to handle the CONFIG_HAVE_IOREMAP_PROT case and + * stack expansion. + */ + *(char *)buf = '\0'; + err = -EFAULT; + goto out; + } + + bytes = len; + offset = addr & (PAGE_SIZE - 1); + if (bytes > PAGE_SIZE - offset) + bytes = PAGE_SIZE - offset; + + maddr = kmap_local_page(page); + retval = strscpy(buf, maddr + offset, bytes); + if (retval >= 0) { + /* Found the end of the string */ + buf += retval; + unmap_and_put_page(page, maddr); + break; + } + + buf += bytes - 1; + /* + * Because strscpy always NUL terminates we need to + * copy the last byte in the page if we are going to + * load more pages + */ + if (bytes != len) { + addr += bytes - 1; + copy_from_user_page(vma, page, addr, buf, maddr + (PAGE_SIZE - 1), 1); + buf += 1; + addr += 1; + } + len -= bytes; + + unmap_and_put_page(page, maddr); + } + +out: + mmap_read_unlock(mm); + if (err) + return err; + return buf - old_buf; +} + +/** + * copy_remote_vm_str - copy a string from another process's address space. + * @tsk: the task of the target address space + * @addr: start address to read from + * @buf: destination buffer + * @len: number of bytes to copy + * @gup_flags: flags modifying lookup behaviour + * + * The caller must hold a reference on @mm. + * + * Return: number of bytes copied from @addr (source) to @buf (destination); + * not including the trailing NUL. Always guaranteed to leave NUL-terminated + * buffer. On any error, return -EFAULT. + */ +int copy_remote_vm_str(struct task_struct *tsk, unsigned long addr, + void *buf, int len, unsigned int gup_flags) +{ + struct mm_struct *mm; + int ret; + + if (unlikely(len == 0)) + return 0; + + mm = get_task_mm(tsk); + if (!mm) { + *(char *)buf = '\0'; + return -EFAULT; + } + + ret = __copy_remote_vm_str(mm, addr, buf, len, gup_flags); + + mmput(mm); + + return ret; +} +EXPORT_SYMBOL_GPL(copy_remote_vm_str); +#endif /* CONFIG_BPF_SYSCALL */ + /* * Print the name of a VMA. */ diff --git a/mm/nommu.c b/mm/nommu.c index baa79abdaf037..0e882a88367a1 100644 --- a/mm/nommu.c +++ b/mm/nommu.c @@ -1708,6 +1708,85 @@ int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, in } EXPORT_SYMBOL_GPL(access_process_vm); +#ifdef CONFIG_BPF_SYSCALL +/* + * Copy a string from another process's address space as given in mm. + * If there is any error return -EFAULT. + */ +static int __copy_remote_vm_str(struct mm_struct *mm, unsigned long addr, + void *buf, int len) +{ + unsigned long addr_end; + struct vm_area_struct *vma; + int ret = -EFAULT; + + *(char *)buf = '\0'; + + if (mmap_read_lock_killable(mm)) + return ret; + + /* the access must start within one of the target process's mappings */ + vma = find_vma(mm, addr); + if (!vma) + goto out; + + if (check_add_overflow(addr, len, &addr_end)) + goto out; + + /* don't overrun this mapping */ + if (addr_end > vma->vm_end) + len = vma->vm_end - addr; + + /* only read mappings where it is permitted */ + if (vma->vm_flags & VM_MAYREAD) { + ret = strscpy(buf, (char *)addr, len); + if (ret < 0) + ret = len - 1; + } + +out: + mmap_read_unlock(mm); + return ret; +} + +/** + * copy_remote_vm_str - copy a string from another process's address space. + * @tsk: the task of the target address space + * @addr: start address to read from + * @buf: destination buffer + * @len: number of bytes to copy + * @gup_flags: flags modifying lookup behaviour (unused) + * + * The caller must hold a reference on @mm. + * + * Return: number of bytes copied from @addr (source) to @buf (destination); + * not including the trailing NUL. Always guaranteed to leave NUL-terminated + * buffer. On any error, return -EFAULT. + */ +int copy_remote_vm_str(struct task_struct *tsk, unsigned long addr, + void *buf, int len, unsigned int gup_flags) +{ + struct mm_struct *mm; + int ret; + + if (unlikely(len == 0)) + return 0; + + mm = get_task_mm(tsk); + if (!mm) { + *(char *)buf = '\0'; + return -EFAULT; + } + + ret = __copy_remote_vm_str(mm, addr, buf, len); + + mmput(mm); + + return ret; +} +EXPORT_SYMBOL_GPL(copy_remote_vm_str); +#endif /* CONFIG_BPF_SYSCALL */ + /** * nommu_shrink_inode_mappings - Shrink the shared mappings on an inode * @inode: The inode to check -- 2.47.2