[XFS] Fix memory corruption with small buffer reads

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>

2 files changed, 39 insertions, 4 deletions

diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
index 5105015a75ad..98e0e86093b4 100644
--- 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);
 
diff --git a/fs/xfs/linux-2.6/xfs_buf.h b/fs/xfs/linux-2.6/xfs_buf.h
index 841d7883528d..f948ec7ba9a4 100644
--- 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 {