[PATCH 5.4 009/389] add barriers to buffer_uptodate and set_buffer_uptodate

Greg Kroah-Hartman
2022-08-23T11:11:16Z

From: Mikulas Patocka <mpatocka@redhat.com>

commit d4252071b97d2027d246f6a82cbee4d52f618b47 upstream.

Let's have a look at this piece of code in __bread_slow:

get_bh(bh);
bh->b_end_io = end_buffer_read_sync;
submit_bh(REQ_OP_READ, 0, bh);
wait_on_buffer(bh);
if (buffer_uptodate(bh))
return bh;

Neither wait_on_buffer nor buffer_uptodate contain any memory barrier.
Consequently, if someone calls sb_bread and then reads the buffer data,
the read of buffer data may be executed before wait_on_buffer(bh) on
architectures with weak memory ordering and it may return invalid data.

Fix this bug by adding a memory barrier to set_buffer_uptodate and an
acquire barrier to buffer_uptodate (in a similar way as
folio_test_uptodate and folio_mark_uptodate).

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/buffer_head.h | 25 ++++++++++++++++++++++++-
1 file changed, 24 insertions(+), 1 deletion(-)

--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -117,7 +117,6 @@ static __always_inline int test_clear_bu
* of the form "mark_buffer_foo()". These are higher-level functions which
* do something in addition to setting a b_state bit.
*/
-BUFFER_FNS(Uptodate, uptodate)
BUFFER_FNS(Dirty, dirty)
TAS_BUFFER_FNS(Dirty, dirty)
BUFFER_FNS(Lock, locked)
@@ -135,6 +134,30 @@ BUFFER_FNS(Meta, meta)
BUFFER_FNS(Prio, prio)
BUFFER_FNS(Defer_Completion, defer_completion)

+static __always_inline void set_buffer_uptodate(struct buffer_head *bh)
+{
+ /*
+ * make it consistent with folio_mark_uptodate
+ * pairs with smp_load_acquire in buffer_uptodate
+ */
+ smp_mb__before_atomic();
+ set_bit(BH_Uptodate, &bh->b_state);
+}
+
+static __always_inline void clear_buffer_uptodate(struct buffer_head *bh)
+{
+ clear_bit(BH_Uptodate, &bh->b_state);
+}
+
+static __always_inline int buffer_uptodate(const struct buffer_head *bh)
+{
+ /*
+ * make it consistent with folio_test_uptodate
+ * pairs with smp_mb__before_atomic in set_buffer_uptodate
+ */
+ return (smp_load_acquire(&bh->b_state) & (1UL << BH_Uptodate)) != 0;
+}
+
#define bh_offset(bh) ((unsigned long)(bh)->b_data & ~PAGE_MASK)

/* If we *know* page->private refers to buffer_heads */