ext4: atomically set inode->i_flags in ext4_set_inode_flags()

Use cmpxchg() to atomically set i_flags instead of clearing out the
S_IMMUTABLE, S_APPEND, etc. flags and then setting them from the
EXT4_IMMUTABLE_FL, EXT4_APPEND_FL flags, since this opens up a race
where an immutable file has the immutable flag cleared for a brief
window of time.

Reported-by: John Sullivan <jsrhbz@kanargh.force9.co.uk>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index abc9ca778456..be5fd38bd5a0 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -196,6 +196,21 @@ static inline unsigned long __ffs64(u64 word)
 
 #ifdef __KERNEL__
 
+#ifndef set_mask_bits
+#define set_mask_bits(ptr, _mask, _bits)        \
+({                                                              \
+        const typeof(*ptr) mask = (_mask), bits = (_bits);      \
+        typeof(*ptr) old, new;                                  \
+                                                                \
+        do {                                                    \
+                old = ACCESS_ONCE(*ptr);                        \
+                new = (old & ~mask) | bits;                     \
+        } while (cmpxchg(ptr, old, new) != old);                \
+                                                                \
+        new;                                                    \
+})
+#endif
+
 #ifndef find_last_bit
 /**
  * find_last_bit - find the last set bit in a memory region