[PATCH] fix crash when using XFS on loopback

From: Mikulas Patocka
Date: Sat Jan 04 2014 - 12:46:09 EST

The patch 8456a648cf44f14365f1f44de90a3da2526a4776 causes crash in the
LVM2 testsuite on PA-RISC (the crashing test is fsadm.sh). The testsuite
doesn't crash on 3.12, crashes on 3.13-rc1 and later.

Bad Address (null pointer deref?): Code=15 regs=000000413edd89a0 (Addr=000006202224647d)
CPU: 3 PID: 24008 Comm: loop0 Not tainted 3.13.0-rc6 #5
task: 00000001bf3c0048 ti: 000000413edd8000 task.ti: 000000413edd8000

PSW: 00001000000001101111100100001110 Not tainted
r00-03 000000ff0806f90e 00000000405c8de0 000000004013e6c0 000000413edd83f0
r04-07 00000000405a95e0 0000000000000200 00000001414735f0 00000001bf349e40
r08-11 0000000010fe3d10 0000000000000001 00000040829c7778 000000413efd9000
r12-15 0000000000000000 000000004060d800 0000000010fe3000 0000000010fe3000
r16-19 000000413edd82a0 00000041078ddbc0 0000000000000010 0000000000000001
r20-23 0008f3d0d83a8000 0000000000000000 00000040829c7778 0000000000000080
r24-27 00000001bf349e40 00000001bf349e40 202d66202224640d 00000000405a95e0
r28-31 202d662022246465 000000413edd88f0 000000413edd89a0 0000000000000001
sr00-03 000000000532c000 0000000000000000 0000000000000000 000000000532c000
sr04-07 0000000000000000 0000000000000000 0000000000000000 0000000000000000

IASQ: 0000000000000000 0000000000000000 IAOQ: 00000000401fe42c 00000000401fe430
IIR: 539c0030 ISR: 00000000202d6000 IOR: 000006202224647d
CPU: 3 CR30: 000000413edd8000 CR31: 0000000000000000
ORIG_R28: 00000000405a95e0
IAOQ[0]: vma_interval_tree_iter_first+0x14/0x48
IAOQ[1]: vma_interval_tree_iter_first+0x18/0x48
RP(r2): flush_dcache_page+0x128/0x388
[<000000004013e6c0>] flush_dcache_page+0x128/0x388
[<0000000010fe6ca0>] lo_splice_actor+0x90/0x148 [loop]
[<00000000402579b0>] splice_from_pipe_feed+0xc0/0x1d0
[<00000000402580a4>] __splice_from_pipe+0xac/0xc0
[<0000000010fe6bbc>] lo_direct_splice_actor+0x1c/0x70 [loop]
[<000000004025854c>] splice_direct_to_actor+0xec/0x228
[<0000000010fe63ac>] lo_receive+0xe4/0x298 [loop]
[<0000000010fe69d8>] loop_thread+0x478/0x640 [loop]
[<000000004018975c>] kthread+0x134/0x168
[<000000004012c020>] end_fault_vector+0x20/0x28
[<00000000115e0098>] xfs_setsize_buftarg+0x0/0x90 [xfs]

Kernel panic - not syncing: Bad Address (null pointer deref?)

The patch 8456a648cf44f14365f1f44de90a3da2526a4776 changes the page
structure so that the slab subsystem reuses the page->mapping field.

The crash happens in the following way:
* XFS allocates some memory from slab and issues a bio to read data into
* the bio is sent to the loopback device.
* lo_receive creates an actor and calls splice_direct_to_actor.
* lo_splice_actor copies data to the target page.
* lo_splice_actor calls flush_dcache_page because the page may be mapped
by userspace. In that case we need to flush the kernel cache.
* flush_dcache_page asks for the list of userspace mappings, however that
page->mapping field is reused by the slab subsystem for a different
purpose. This causes the crash.

Note that other architectures without coherent caches (sparc, arm, mips)
also call page_mapping from flush_dcache_page, so they may crash in the
same way.

This patch fixes this bug by testing if the page is a slab page in
page_mapping and returning NULL if it is.

The patch also fixes VM_BUG_ON(PageSlab(page)) that could happen in
earlier kernels in the same scenario on architectures without cache
coherence when CONFIG_DEBUG_VM is enabled - so it should be backported to
stable kernels.

In the old kernels, the function page_mapping is placed in
include/linux/mm.h, so you should modify the patch accordingly when
backporting it.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx

mm/util.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

Index: linux-3.13-rc6/mm/util.c
--- linux-3.13-rc6.orig/mm/util.c 2014-01-04 00:06:07.000000000 +0100
+++ linux-3.13-rc6/mm/util.c 2014-01-04 00:24:42.000000000 +0100
@@ -390,7 +390,10 @@ struct address_space *page_mapping(struc
struct address_space *mapping = page->mapping;

- VM_BUG_ON(PageSlab(page));
+ /* This happens if someone calls flush_dcache_page on slab page */
+ if (unlikely(PageSlab(page)))
+ return NULL;
if (unlikely(PageSwapCache(page))) {
swp_entry_t entry;

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/