Re: [PATCH] kfence: unpoison pool region before use

Marco Elver
2021-04-03T22:37:49Z

On Sat, 3 Apr 2021 at 22:40, Peter Collingbourne <pcc@google.com> wrote:
> On Sat, Apr 3, 2021 at 3:03 AM Marco Elver <elver@google.com> wrote:
> > On Sat, 3 Apr 2021 at 07:13, Peter Collingbourne <pcc@google.com> wrote:
> > > If the memory region allocated by KFENCE had previously been poisoned,
> > > any validity checks done using kasan_byte_accessible() will fail. Fix
> > > it by unpoisoning the memory before using it as the pool region.
> > >
> > > Link: https://linux-review.googlesource.com/id/I0af99e9f1c25eaf7e1ec295836b5d148d76940c5
> > > Signed-off-by: Peter Collingbourne <pcc@google.com>
> >
> > Thanks, at a high level this seems reasonable, because we always want
> > to ensure that KFENCE memory remains unpoisoned with KASAN on. FWIW I
> > subjected a config with KFENCE+KASAN (generic, SW_TAGS, and HW_TAGS)
> > to syzkaller testing and ran kfence_test:
> >
> > Tested-by: Marco Elver <elver@google.com>
> >
> >
> > However, it is unclear to me under which circumstances we actually
> > need this, i.e. something would grab some memblock memory, somehow
> > poison it, and then release the memory back during early boot (note,
> > kfence_alloc_pool() is called before slab setup). If we can somehow
> > understand what actually did this, perhaps it'd help tell us if this
> > actually needs fixing in KFENCE or it's the other thing that needs a
> > fix.
> >
> > Given all this is happening during really early boot, I'd expect no or
> > very few calls to kasan_poison() until kfence_alloc_pool() is called.
> > We can probably debug it more by having kasan_poison() do a "if
> > (!__kfence_pool) dump_stack();" somewhere. Can you try this on the
> > system where you can repro the problem? I tried this just now on the
> > latest mainline kernel, and saw 0 calls until kfence_alloc_pool().
>
> I looked into the issue some more, and it turned out that the memory
> wasn't getting poisoned by kasan_poison() but rather by the calls to
> kasan_map_populate() in kasan_init_shadow(). Starting with the patch
> "kasan: initialize shadow to TAG_INVALID for SW_TAGS",
> KASAN_SHADOW_INIT is set to 0xFE rather than 0xFF, which caused the
> failure. The Android kernel branch for 5.10 (and the downstream kernel
> I was working with) already have this patch, but it isn't in the
> mainline kernel yet.
>
> Now that I understand the cause of the issue, I can reproduce it using
> the KFENCE unit tests on a db845c board, using both the Android 5.10
> and mainline branches if I cherry-pick that change. Here's an example
> crash from the unit tests (the failure was originally also observed
> from ksize in the downstream kernel):
>
> [ 46.692195][ T175] BUG: KASAN: invalid-access in test_krealloc+0x1c4/0xf98
> [ 46.699282][ T175] Read of size 1 at addr ffffff80e9e7b000 by task
> kunit_try_catch/175
> [ 46.707400][ T175] Pointer tag: [ff], memory tag: [fe]
> [ 46.712710][ T175]
> [ 46.714955][ T175] CPU: 4 PID: 175 Comm: kunit_try_catch Tainted:
> G B 5.12.0-rc5-mainline-09505-ga2ab5b26d445-dirty #1
> [ 46.727193][ T175] Hardware name: Thundercomm Dragonboard 845c (DT)
> [ 46.733636][ T175] Call trace:
> [ 46.736841][ T175] dump_backtrace+0x0/0x2f8
> [ 46.741295][ T175] show_stack+0x2c/0x3c
> [ 46.745388][ T175] dump_stack+0x124/0x1bc
> [ 46.749668][ T175] print_address_description+0x7c/0x308
> [ 46.755178][ T175] __kasan_report+0x1a8/0x398
> [ 46.759816][ T175] kasan_report+0x50/0x7c
> [ 46.764103][ T175] __kasan_check_byte+0x3c/0x54
> [ 46.768916][ T175] ksize+0x4c/0x94
> [ 46.772573][ T175] test_krealloc+0x1c4/0xf98
> [ 46.777108][ T175] kunit_try_run_case+0x94/0x1c4
> [ 46.781990][ T175] kunit_generic_run_threadfn_adapter+0x30/0x44
> [ 46.788196][ T175] kthread+0x20c/0x234
> [ 46.792213][ T175] ret_from_fork+0x10/0x30
>
> Since "kasan: initialize shadow to TAG_INVALID for SW_TAGS" hasn't
> landed in mainline yet, it seems like we should insert this patch
> before that one rather than adding a Fixes: tag.

Thanks for getting to the bottom of it.

However, given the above, I think we need to explain this in the
commit message (which also makes the dependency between these 2
patches clear) and add a comment above the new kasan_unpoison_range().
That is, if we still think this is the right fix -- I'm not entirely
sure it is.

Because what I gather from "kasan: initialize shadow to TAG_INVALID
for SW_TAGS", is the requirement that "0xFF pointer tag is a match-all
tag, it doesn't matter what tag the accessed memory has".

While KFENCE memory is accessible through the slab API, and in this
case ksize() calling kasan_check_byte() leading to a failure, the
kasan_check_byte() call is part of the public KASAN API. Which means
that if some subsystem decides to memblock_alloc() some memory, and
wishes to use kasan_check_byte() on that memory but with an untagged
pointer, will get the same problem as KFENCE: with generic and HW_TAGS
mode everything is fine, but with SW_TAGS mode things break.

To me this indicates the fix is not with KFENCE, but should be in
mm/kasan/sw_tags.c:kasan_byte_accessible(), which should not load the
shadow when the pointer is untagged.

Thanks,
-- Marco