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 <pfalcato@suse.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christian Brauner (Amutable) <brauner@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/fs.h b/include/linux/fs.h
index d10897b3a1e35017d0286a822106f5fd4b0d7edd..50ce731a2b78f140b1990ee49b976a8d2af05f50 100644 (file)
--- 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 5af62e6abca58b04f038466d3736cf91d8dbca81..58eb9d24064348020b3b706b555729587adbc805 100644 (file)
--- 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 cd9bb077072ccb1b90242a3211a9c6ef48929f50..77552b03d318d9ab93902cd42670dd23f26d83a7 100644 (file)
--- 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 296f2e3922b5f48eb66a2c54ca0e69cc2e91c1a1..c8757c5085bf90040aa135433385ecea9f97a234 100644 (file)
--- 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;
 }