lkml.org 
[lkml]   [2021]   [Dec]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH v2 6/9] refcount: Fix refcount_dec_not_one()
refcount_dec_not_one() misbehaves when racing against the other
refcount ops, notably it directly compares against REFCOUNT_SATURATED,
even though, through aforementioned races, the value might be slightly
off.

Additionally, when refcount_dec_not_one() is used to decrement zero;
which is valid, because 0 is not one, then it behaves differently from
the other refcount_dec*() functions in that it doesn't call
refcount_warn_saturate().

In order to allow some fuzz around the corners of the positive range,
test for saturation slightly differently.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
lib/refcount.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

--- a/lib/refcount.c
+++ b/lib/refcount.c
@@ -81,7 +81,8 @@ bool refcount_dec_not_one(refcount_t *r)
unsigned int new, val = atomic_read(&r->refs);

do {
- if (unlikely(val == REFCOUNT_SATURATED))
+ /* SAT+SAT/2 < val < SAT-SAT/2 */
+ if (unlikely(val - (REFCOUNT_SATURATED + REFCOUNT_SATURATED/2) < -REFCOUNT_SATURATED))
return true;

if (val == 1)
@@ -89,7 +90,7 @@ bool refcount_dec_not_one(refcount_t *r)

new = val - 1;
if (new > val) {
- WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
+ refcount_warn_saturate(r, REFCOUNT_SUB_UAF);
return true;
}


\
 
 \ /
  Last update: 2021-12-10 17:28    [W:0.162 / U:0.708 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site