[PATCH] mm: page_counter: reject empty string in page_counter_memparse()

From: Tao Cui <[email protected]>

memparse() consumes no characters on an empty input and leaves the
end pointer at the terminating NUL.  The only validation in
page_counter_memparse() checks for trailing characters, so an empty
input slips through and the limit becomes 0.

All limit write callbacks of the memory controller strstrip() the
input before calling this helper, so a script that writes an unset
variable hits this path:

  LIMIT=
  echo "$LIMIT" > $CG/memory.max
  echo $?
  0
  cat $CG/memory.max
  0

Nothing reports the mistake: the limit is now 0 and the OOM killer
goes after every task in the cgroup.  The same happens for
memory.min, memory.low, memory.high, memory.swap.high,
memory.swap.max and memory.zswap.max, where 0 silently removes the
protection or disables swap and zswap.

Reject the input when no characters were consumed, which is the one
case the trailing-character check cannot catch.

Fixes: 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
Signed-off-by: Tao Cui <[email protected]>
---
 mm/page_counter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_counter.c b/mm/page_counter.c
index 661e0f2a5127..d14db705b04f 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -281,7 +281,7 @@ int page_counter_memparse(const char *buf, const char *max,
 	}
 
 	bytes = memparse(buf, &end);
-	if (*end != '\0')
+	if (*end != '\0' || end == buf)
 		return -EINVAL;
 
 	*nr_pages = min(bytes / PAGE_SIZE, (u64)PAGE_COUNTER_MAX);
-- 
2.43.0