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blk-mq: Clear out elevator private data

PATCH In review Kees Cook 3 participants 5 messages Apr 17, 2018 – Apr 18, 2018

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Kees Cook

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#0 lore raw

PATCH 1/1 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 <[email protected]>
Fixes bd166ef183c26 ("blk-mq-sched: add framework for MQ capable IO schedulers")
Cc [email protected]
Signed-off-by Kees Cook <[email protected]>

---
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(-)
block/blk-mq.c +2 -2
index 0dc9e341c2a7..859df3160303 100644
@@ -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));
 		}
 	}
Signature · 6 lines
2.7.4


-- 
Kees Cook
Pixel Security
maintainer
#1 lore raw
Quoted 10 lines · Kees Cook

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.

Comments from the other email still apply, we should not need to do this full memset() for every request. From a quick look, BFQ needs to straighten out its usage of prepare request and interactions with insert_request.

Quoted 8 lines
Reported-by Oleksandr Natalenko <[email protected]>
Fixes bd166ef183c26 ("blk-mq-sched: add framework for MQ capable IO schedulers")
Cc [email protected]
Signed-off-by Kees Cook <[email protected]>

---
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.

Yeah, that's probably a bit too broad for fixes :-)

Signature · 1 line
Jens Axboe
author
#2 lore raw
Quoted 8 lines · Jens Axboe
Quoted 7 lines · Kees Cook

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.

Quoted 7 lines
Quoted 3 lines

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.

Comments from the other email still apply, we should not need to do this full memset() for every request. From a quick look, BFQ needs to straighten out its usage of prepare request and interactions with insert_request.

Sure, understood. I would point out, FWIW, that memset() gets unrolled by the compiler and this is just two more XORs in the same cacheline (the two words following icq). (And there is SO much more being cleared during alloc, it didn't seem like hardly any extra cost vs the robustness it provided.)

-Kees

Signature · 2 lines
Kees Cook
Pixel Security
maintainer
#3 lore raw
Quoted 17 lines · Kees Cook
Quoted 8 lines · Jens Axboe
Quoted 7 lines · Kees Cook

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.

Quoted 7 lines
Quoted 3 lines

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.

Comments from the other email still apply, we should not need to do this full memset() for every request. From a quick look, BFQ needs to straighten out its usage of prepare request and interactions with insert_request.

Quoted 5 lines

Sure, understood. I would point out, FWIW, that memset() gets unrolled by the compiler and this is just two more XORs in the same cacheline (the two words following icq). (And there is SO much more being cleared during alloc, it didn't seem like hardly any extra cost vs the robustness it provided.)

Yeah, it's not super pricey, but it's not needed. BFQ is the user of the members, and the one that assigns them. You're saying leftover assignments, since it doesn't always assign them. Hence I think that's a better fix, just sent out a test patch a few minutes ago.

You did all the hard work, I'm just coasting on your findings.

Signature · 1 line
Jens Axboe
#4 lore raw
In reply to Kees Cook
Quoted 3 lines

Il giorno 17 apr 2018, alle ore 23:42, Kees Cook =

<[email protected]> ha scritto:

Quoted 4 lines

=20
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.

Hi Kees,
where does BFQ gets confused and operates on a request not destined to it? I'm asking because I paid attention to always avoid such a mistake.

Thanks,
Paolo

Quoted 7 lines

This could lead to odd behaviors like having the sense buffer address slowly start incrementing. This eventually tripped HARDENED_USERCOPY and KASAN.
=20
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.

Quoted 3 lines

=20

Reported-by Oleksandr Natalenko <[email protected]>
Fixes bd166ef183c26 ("blk-mq-sched: add framework for MQ capable IO =

schedulers")

Quoted 4 lines
Cc [email protected]
Signed-off-by Kees Cook <[email protected]>

---
In theory, BFQ needs to also check the RQF_ELVPRIV flag, but I'll =

leave that

Quoted 1 line

to Paolo to figure out. Also, my Fixes line is kind of a best-guess. =

This

Quoted 1 line

is where icq was originally wiped, so it seemed as good a commit as =

any.

block/blk-mq.c static struct request *blk_mq_get_request(struct =

Quoted 9 lines

---
block/blk-mq.c | 4 ++--

1 file changed, 2 insertions(+), 2 deletions(-)

=20

block/blk-mq.c +0 -0
index 0dc9e341c2a7..859df3160303 100644
@@ -363,7 +363,7 @@ static struct request *blk_mq_get_request(struct =

request_queue *q,

Quoted 24 lines

=20

	rq =3D blk_mq_rq_ctx_init(data, tag, op);
	if (!op_is_flush(op)) {
block/blk-mq.c +1 -1
-		rq->elv.icq =3D 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);
block/blk-mq.c +1 -1
@@ -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 =3D NULL;
+			memset(&rq->elv, 0, sizeof(rq->elv));
 		}
 	}

=20
--=20
2.7.4
=20
=20
--=20
Kees Cook
Pixel Security