From: Pedro Falcato Date: Thu, 25 Jun 2026 15:38:53 +0000 (+0100) Subject: mm: do file ownership checks with the proper mount idmap X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=e187bc02f8fa4226d62814592cf064ee4557c470;p=thirdparty%2Flinux.git mm: do file ownership checks with the proper mount idmap Ever since idmapped mounts were introduced, inode ownership checks (for side-channel protection) in mincore() and madvise(MADV_PAGEOUT) were done against the nop_mnt_idmap, which completely ignores the file's mount's idmap. This results in odd edgecases like: 1) mount/bind-mount with an idmap userA:userB:1 2) userB runs an owner_or_capable() check on file that is owned by userA on-disk/in-memory, but owned by userB after idmap translation 3) owner_or_capable() mysteriously fails as the correct idmap wasn't supplied In the case of mincore/madvise MADV_PAGEOUT, this is usually benign, because file_permission(file, MAY_WRITE) will probably succeed, as it uses the proper idmap internally, but it does not need to be the case on e.g a 0444 file where even the owner itself doesn't have permissions to write to it. Since this is clearly not trivial to get right, introduce a file_owner_or_capable() that can carry the correct semantics, and switch the various users in mm to it. The issue was found by manual code inspection & an off-list discussion with Jan Kara. Link: https://lore.kernel.org/20260625153853.913949-1-pfalcato@suse.de Fixes: 9caccd41541a ("fs: introduce MOUNT_ATTR_IDMAP") Signed-off-by: Pedro Falcato Reviewed-by: Jan Kara Reviewed-by: Christian Brauner (Amutable) Acked-by: David Hildenbrand (Arm) Cc: Al Viro Cc: Jann Horn Cc: Liam R. Howlett Cc: Matthew Wilcox (Oracle) Cc: Vlastimil Babka Cc: Signed-off-by: Andrew Morton --- diff --git a/include/linux/fs.h b/include/linux/fs.h index d10897b3a1e3..50ce731a2b78 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2444,6 +2444,11 @@ static inline struct mnt_idmap *file_mnt_idmap(const struct file *file) return mnt_idmap(file->f_path.mnt); } +static inline bool file_owner_or_capable(const struct file *file) +{ + return inode_owner_or_capable(file_mnt_idmap(file), file_inode(file)); +} + /** * is_idmapped_mnt - check whether a mount is mapped * @mnt: the mount to check diff --git a/mm/filemap.c b/mm/filemap.c index 5af62e6abca5..58eb9d240643 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -4704,7 +4704,7 @@ static inline bool can_do_cachestat(struct file *f) { if (f->f_mode & FMODE_WRITE) return true; - if (inode_owner_or_capable(file_mnt_idmap(f), file_inode(f))) + if (file_owner_or_capable(f)) return true; return file_permission(f, MAY_WRITE) == 0; } diff --git a/mm/madvise.c b/mm/madvise.c index cd9bb077072c..77552b03d318 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -336,8 +336,7 @@ static inline bool can_do_file_pageout(struct vm_area_struct *vma) * otherwise we'd be including shared non-exclusive mappings, which * opens a side channel. */ - return inode_owner_or_capable(&nop_mnt_idmap, - file_inode(vma->vm_file)) || + return file_owner_or_capable(vma->vm_file) || file_permission(vma->vm_file, MAY_WRITE) == 0; } diff --git a/mm/mincore.c b/mm/mincore.c index 296f2e3922b5..c8757c5085bf 100644 --- a/mm/mincore.c +++ b/mm/mincore.c @@ -227,8 +227,7 @@ static inline bool can_do_mincore(struct vm_area_struct *vma) * for writing; otherwise we'd be including shared non-exclusive * mappings, which opens a side channel. */ - return inode_owner_or_capable(&nop_mnt_idmap, - file_inode(vma->vm_file)) || + return file_owner_or_capable(vma->vm_file) || file_permission(vma->vm_file, MAY_WRITE) == 0; }