[tip: locking/urgent] futex: Allow to resize the private local hash

["tip-bot2 for Sebastian Andrzej Siewior" <tip-bot2@linutronix.de>]

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     cdd0f803c1f9b69785f5ff865864cfea11081c91
Gitweb:        https://git.kernel.org/tip/cdd0f803c1f9b69785f5ff865864cfea11081c91
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Mon, 02 Jun 2025 13:00:27 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 05 Jun 2025 14:37:59 +02:00

futex: Allow to resize the private local hash

On 2025-06-01 15:39:47 [+0800], Lai, Yi wrote:
> Hi Sebastian Andrzej Siewior,
Hi Yi,
> Greetings!
>
> I used Syzkaller and found that there is KASAN: null-ptr-deref Read in __futex_pivot_hash in linux-next next-20250527.
>
> After bisection and the first bad commit is:
> "
> bd54df5ea7ca futex: Allow to resize the private local hash
> "

Thank you for the report. Next time please trim your report. There is no
need to put your report in the middle of the patch.

The following fixes it:

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250602110027.wfqbHgzb@linutronix.de
---
 kernel/futex/core.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index b652d2f..33b3643 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1629,6 +1629,16 @@ again:
 		mm->futex_phash_new = NULL;
 
 		if (fph) {
+			if (cur && !cur->hash_mask) {
+				/*
+				 * If two threads simultaneously request the global
+				 * hash then the first one performs the switch,
+				 * the second one returns here.
+				 */
+				free = fph;
+				mm->futex_phash_new = new;
+				return -EBUSY;
+			}
 			if (cur && !new) {
 				/*
 				 * If we have an existing hash, but do not yet have