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Subject[PATCH 5.4 164/177] mm, memfd: fix COW issue on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings
Date
From: Nicolas Geoffray <ngeoffray@google.com>

commit 05d351102dbe4e103d6bdac18b1122cd3cd04925 upstream.

F_SEAL_FUTURE_WRITE has unexpected behavior when used with MAP_PRIVATE:
A private mapping created after the memfd file that gets sealed with
F_SEAL_FUTURE_WRITE loses the copy-on-write at fork behavior, meaning
children and parent share the same memory, even though the mapping is
private.

The reason for this is due to the code below:

static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
{
struct shmem_inode_info *info = SHMEM_I(file_inode(file));

if (info->seals & F_SEAL_FUTURE_WRITE) {
/*
* New PROT_WRITE and MAP_SHARED mmaps are not allowed when
* "future write" seal active.
*/
if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE))
return -EPERM;

/*
* Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED
* read-only mapping, take care to not allow mprotect to revert
* protections.
*/
vma->vm_flags &= ~(VM_MAYWRITE);
}
...
}

And for the mm to know if a mapping is copy-on-write:

static inline bool is_cow_mapping(vm_flags_t flags)
{
return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
}

The patch fixes the issue by making the mprotect revert protection
happen only for shared mappings. For private mappings, using mprotect
will have no effect on the seal behavior.

The F_SEAL_FUTURE_WRITE feature was introduced in v5.1 so v5.3.x stable
kernels would need a backport.

[akpm@linux-foundation.org: reflow comment, per Christoph]
Link: http://lkml.kernel.org/r/20191107195355.80608-1-joel@joelfernandes.org
Fixes: ab3948f58ff84 ("mm/memfd: add an F_SEAL_FUTURE_WRITE seal to memfd")
Signed-off-by: Nicolas Geoffray <ngeoffray@google.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/shmem.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2213,11 +2213,14 @@ static int shmem_mmap(struct file *file,
return -EPERM;

/*
- * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED
- * read-only mapping, take care to not allow mprotect to revert
- * protections.
+ * Since an F_SEAL_FUTURE_WRITE sealed memfd can be mapped as
+ * MAP_SHARED and read-only, take care to not allow mprotect to
+ * revert protections on such mappings. Do this only for shared
+ * mappings. For private mappings, don't need to mask
+ * VM_MAYWRITE as we still want them to be COW-writable.
*/
- vma->vm_flags &= ~(VM_MAYWRITE);
+ if (vma->vm_flags & VM_SHARED)
+ vma->vm_flags &= ~(VM_MAYWRITE);
}

file_accessed(file);

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