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authorNicolas Geoffray <ngeoffray@google.com>2019-11-30 17:53:28 -0800
committerPaul Gortmaker <paul.gortmaker@windriver.com>2020-01-31 16:51:01 -0500
commitfbac2f7aa1c32de6797559ae553aaaef150d5cb1 (patch)
tree0f3471676de5f92ce1798b8cc71d147471468171
parent11819775668320f4d62946abea5aec0259155f85 (diff)
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mm, memfd: fix COW issue on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings
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: Paul Gortmaker <paul.gortmaker@windriver.com>
-rw-r--r--mm/shmem.c11
1 files changed, 7 insertions, 4 deletions
diff --git a/mm/shmem.c b/mm/shmem.c
index f4dce9c8670d..b093b429d325 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2196,11 +2196,14 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
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);