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Subject[PATCH] blk-mq: Clear out elevator private data
Some elevators may not correctly check rq->rq_flags & RQF_ELVPRIV, and
may attempt to read rq->elv fields. When requests got reused, this
caused BFQ to think it already had a bfqq (rq->elv.priv[1]) allocated.
This could lead to odd behaviors like having the sense buffer address
slowly start incrementing. This eventually tripped HARDENED_USERCOPY
and KASAN.

This patch wipes all of rq->elv instead of just rq->elv.icq. While
it shouldn't technically be needed, this ends up being a robustness
improvement that should lead to at least finding bugs in elevators faster.

Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Fixes: bd166ef183c26 ("blk-mq-sched: add framework for MQ capable IO schedulers")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
In theory, BFQ needs to also check the RQF_ELVPRIV flag, but I'll leave that
to Paolo to figure out. Also, my Fixes line is kind of a best-guess. This
is where icq was originally wiped, so it seemed as good a commit as any.
---
block/blk-mq.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0dc9e341c2a7..859df3160303 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -363,7 +363,7 @@ static struct request *blk_mq_get_request(struct request_queue *q,

rq = blk_mq_rq_ctx_init(data, tag, op);
if (!op_is_flush(op)) {
- rq->elv.icq = NULL;
+ memset(&rq->elv, 0, sizeof(rq->elv));
if (e && e->type->ops.mq.prepare_request) {
if (e->type->icq_cache && rq_ioc(bio))
blk_mq_sched_assign_ioc(rq, bio);
@@ -461,7 +461,7 @@ void blk_mq_free_request(struct request *rq)
e->type->ops.mq.finish_request(rq);
if (rq->elv.icq) {
put_io_context(rq->elv.icq->ioc);
- rq->elv.icq = NULL;
+ memset(&rq->elv, 0, sizeof(rq->elv));
}
}

--
2.7.4

--
Kees Cook
Pixel Security

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