[PATCH] mm/compaction: fix an undefined behaviour

[PATCH] mm/compaction: fix an undefined behaviour

[Qian Cai]

In a low-memory situation, cc->fast_search_fail can keep increasing as
it is unable to find an available page to isolate in
fast_isolate_freepages(). As the result, it could trigger an error
below, so just compare with the maximum bits can be shifted first.

UBSAN: Undefined behaviour in mm/compaction.c:1160:30
shift exponent 64 is too large for 64-bit type 'unsigned long'
CPU: 131 PID: 1308 Comm: kcompactd1 Kdump: loaded Tainted: G
W    L    5.0.0+ #17
Call trace:
 dump_backtrace+0x0/0x450
 show_stack+0x20/0x2c
 dump_stack+0xc8/0x14c
 __ubsan_handle_shift_out_of_bounds+0x7e8/0x8c4
 compaction_alloc+0x2344/0x2484
 unmap_and_move+0xdc/0x1dbc
 migrate_pages+0x274/0x1310
 compact_zone+0x26ec/0x43bc
 kcompactd+0x15b8/0x1a24
 kthread+0x374/0x390
 ret_from_fork+0x10/0x18

Fixes: 70b44595eafe ("mm, compaction: use free lists to quickly locate a migration source")
Signed-off-by: Qian Cai <cai@lca.pw>
---
 mm/compaction.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index f171a83707ce..6aebf1eb8d98 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1157,7 +1157,9 @@ static bool suitable_migration_target(struct compact_control *cc,
 static inline unsigned int
 freelist_scan_limit(struct compact_control *cc)
 {
-	return (COMPACT_CLUSTER_MAX >> cc->fast_search_fail) + 1;
+	return (COMPACT_CLUSTER_MAX >>
+		min((unsigned short)(BITS_PER_LONG - 1), cc->fast_search_fail))
+		+ 1;
 }
 
 /*
-- 
2.17.2 (Apple Git-113)

Re: [PATCH] mm/compaction: fix an undefined behaviour

[Mel Gorman]

On Fri, Mar 08, 2019 at 05:46:50PM -0500, Qian Cai wrote:
> In a low-memory situation, cc->fast_search_fail can keep increasing as
> it is unable to find an available page to isolate in
> fast_isolate_freepages(). As the result, it could trigger an error
> below, so just compare with the maximum bits can be shifted first.
> 
> UBSAN: Undefined behaviour in mm/compaction.c:1160:30
> shift exponent 64 is too large for 64-bit type 'unsigned long'
> CPU: 131 PID: 1308 Comm: kcompactd1 Kdump: loaded Tainted: G
> W    L    5.0.0+ #17
> Call trace:
>  dump_backtrace+0x0/0x450
>  show_stack+0x20/0x2c
>  dump_stack+0xc8/0x14c
>  __ubsan_handle_shift_out_of_bounds+0x7e8/0x8c4
>  compaction_alloc+0x2344/0x2484
>  unmap_and_move+0xdc/0x1dbc
>  migrate_pages+0x274/0x1310
>  compact_zone+0x26ec/0x43bc
>  kcompactd+0x15b8/0x1a24
>  kthread+0x374/0x390
>  ret_from_fork+0x10/0x18
> 
> Fixes: 70b44595eafe ("mm, compaction: use free lists to quickly locate a migration source")
> Signed-off-by: Qian Cai <cai@lca.pw>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

FWIW, I had seen the same message when trying to isolate potential
corruption (still unsuccessful, the tests always complete) but
considered it relatively benign and harmless. Still worth fixing so
thanks.

-- 
Mel Gorman
SUSE Labs