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Subject[patch 29/50] XFS: Fix memory corruption with small buffer reads
-stable review patch.  If anyone has any objections, please let us know.
---------------------

From: Christoph Hellwig <hch@infradead.org>

upstream commit: 6ab455eeaff6893cd06da33843e840d888cdc04a

When we have multiple buffers in a single page for a blocksize == pagesize
filesystem we might overwrite the page contents if two callers hit it
shortly after each other. To prevent that we need to keep the page locked
until I/O is completed and the page marked uptodate.

Thanks to Eric Sandeen for triaging this bug and finding a reproducible
testcase and Dave Chinner for additional advice.

This should fix kernel.org bz #10421.

Tested-by: Eric Sandeen <sandeen@sandeen.net>

SGI-PV: 981813
SGI-Modid: xfs-linux-melb:xfs-kern:31173a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
---
fs/xfs/linux-2.6/xfs_buf.c | 24 ++++++++++++++++++++----
fs/xfs/linux-2.6/xfs_buf.h | 19 +++++++++++++++++++
2 files changed, 39 insertions(+), 4 deletions(-)

--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -387,6 +387,8 @@ _xfs_buf_lookup_pages(
if (unlikely(page == NULL)) {
if (flags & XBF_READ_AHEAD) {
bp->b_page_count = i;
+ for (i = 0; i < bp->b_page_count; i++)
+ unlock_page(bp->b_pages[i]);
return -ENOMEM;
}

@@ -416,17 +418,24 @@ _xfs_buf_lookup_pages(
ASSERT(!PagePrivate(page));
if (!PageUptodate(page)) {
page_count--;
- if (blocksize < PAGE_CACHE_SIZE && !PagePrivate(page)) {
+ if (blocksize >= PAGE_CACHE_SIZE) {
+ if (flags & XBF_READ)
+ bp->b_flags |= _XBF_PAGE_LOCKED;
+ } else if (!PagePrivate(page)) {
if (test_page_region(page, offset, nbytes))
page_count++;
}
}

- unlock_page(page);
bp->b_pages[i] = page;
offset = 0;
}

+ if (!(bp->b_flags & _XBF_PAGE_LOCKED)) {
+ for (i = 0; i < bp->b_page_count; i++)
+ unlock_page(bp->b_pages[i]);
+ }
+
if (page_count == bp->b_page_count)
bp->b_flags |= XBF_DONE;

@@ -746,6 +755,7 @@ xfs_buf_associate_memory(
bp->b_count_desired = len;
bp->b_buffer_length = buflen;
bp->b_flags |= XBF_MAPPED;
+ bp->b_flags &= ~_XBF_PAGE_LOCKED;

return 0;
}
@@ -1093,8 +1103,10 @@ _xfs_buf_ioend(
xfs_buf_t *bp,
int schedule)
{
- if (atomic_dec_and_test(&bp->b_io_remaining) == 1)
+ if (atomic_dec_and_test(&bp->b_io_remaining) == 1) {
+ bp->b_flags &= ~_XBF_PAGE_LOCKED;
xfs_buf_ioend(bp, schedule);
+ }
}

STATIC void
@@ -1125,6 +1137,9 @@ xfs_buf_bio_end_io(

if (--bvec >= bio->bi_io_vec)
prefetchw(&bvec->bv_page->flags);
+
+ if (bp->b_flags & _XBF_PAGE_LOCKED)
+ unlock_page(page);
} while (bvec >= bio->bi_io_vec);

_xfs_buf_ioend(bp, 1);
@@ -1163,7 +1178,8 @@ _xfs_buf_ioapply(
* filesystem block size is not smaller than the page size.
*/
if ((bp->b_buffer_length < PAGE_CACHE_SIZE) &&
- (bp->b_flags & XBF_READ) &&
+ ((bp->b_flags & (XBF_READ|_XBF_PAGE_LOCKED)) ==
+ (XBF_READ|_XBF_PAGE_LOCKED)) &&
(blocksize >= PAGE_CACHE_SIZE)) {
bio = bio_alloc(GFP_NOIO, 1);

--- a/fs/xfs/linux-2.6/xfs_buf.h
+++ b/fs/xfs/linux-2.6/xfs_buf.h
@@ -66,6 +66,25 @@ typedef enum {
_XBF_PAGES = (1 << 18), /* backed by refcounted pages */
_XBF_RUN_QUEUES = (1 << 19),/* run block device task queue */
_XBF_DELWRI_Q = (1 << 21), /* buffer on delwri queue */
+
+ /*
+ * Special flag for supporting metadata blocks smaller than a FSB.
+ *
+ * In this case we can have multiple xfs_buf_t on a single page and
+ * need to lock out concurrent xfs_buf_t readers as they only
+ * serialise access to the buffer.
+ *
+ * If the FSB size >= PAGE_CACHE_SIZE case, we have no serialisation
+ * between reads of the page. Hence we can have one thread read the
+ * page and modify it, but then race with another thread that thinks
+ * the page is not up-to-date and hence reads it again.
+ *
+ * The result is that the first modifcation to the page is lost.
+ * This sort of AGF/AGI reading race can happen when unlinking inodes
+ * that require truncation and results in the AGI unlinked list
+ * modifications being lost.
+ */
+ _XBF_PAGE_LOCKED = (1 << 22),
} xfs_buf_flags_t;

typedef enum {
--


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