From: Alexander Popov <alex.popov@linux.com> To: Mark Rutland <mark.rutland@arm.com> Cc: Andy Lutomirski <luto@kernel.org>, Laura Abbott <labbott@redhat.com>, Kees Cook <keescook@chromium.org>, Ard Biesheuvel <ard.biesheuvel@linaro.org>, kernel-hardening@lists.openwall.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] arm64: Clear the stack Date: Mon, 14 May 2018 16:53:12 +0300 [thread overview] Message-ID: <c5bebf0b-9667-5725-2800-97c5a85635c4@linux.com> (raw) In-Reply-To: <20180514100639.v3erlzbuv2e4awfh@lakrids.cambridge.arm.com> On 14.05.2018 13:06, Mark Rutland wrote: > On Mon, May 14, 2018 at 12:35:25PM +0300, Alexander Popov wrote: >> On 14.05.2018 08:15, Mark Rutland wrote: >>> On Sun, May 13, 2018 at 11:40:07AM +0300, Alexander Popov wrote: >>>> So what would you think if I do the following in check_alloca(): >>>> >>>> if (size >= stack_left) { >>>> #if !defined(CONFIG_VMAP_STACK) && defined(CONFIG_SCHED_STACK_END_CHECK) >>>> panic("alloca over the kernel stack boundary\n"); >>>> #else >>>> BUG(); >>>> #endif >>> >>> Given this is already out-of-line, how about we always use panic(), regardless >>> of VMAP_STACK and SCHED_STACK_END_CHECK? i.e. just >>> >>> if (unlikely(size >= stack_left)) >>> panic("alloca over the kernel stack boundary"); >>> >>> If we have VMAP_STACK selected, and overflow during the panic, it's the same as >>> if we overflowed during the BUG(). It's likely that panic() will use less stack >>> space than BUG(), and the compiler can put the call in a slow path that >>> shouldn't affect most calls, so in all cases it's likely preferable. >> >> I'm sure that maintainers and Linus will strongly dislike my patch if I always >> use panic() here. panic() kills the whole kernel and we shouldn't use it when we >> can safely continue to work. >> >> Let me describe my logic. So let's have size >= stack_left on a thread stack. >> >> 1. If CONFIG_VMAP_STACK is enabled, we can safely use BUG(). Even if BUG() >> handling overflows the thread stack into the guard page, handle_stack_overflow() >> is called and the neighbour memory is not corrupted. The kernel can proceed to live. > > On arm64 with CONFIG_VMAP_STACK, a stack overflow will result in a > panic(). My understanding was that the same is true on x86. No, x86 CONFIG_VMAP_STACK only kills the offending process. I see it on my deep recursion test, the kernel continues to live. handle_stack_overflow() in arch/x86/kernel/traps.c calls die(). >> 2. If CONFIG_VMAP_STACK is disabled, BUG() handling can corrupt the neighbour >> kernel memory and cause the undefined behaviour of the whole kernel. I see it on >> my lkdtm test. That is a cogent reason for panic(). > > In this case, panic() can also corrupt the neighbour stack, and could > also fail. > > When CONFIG_VMAP_STACK is not selected, a stack overflow simply cannot > be handled reliably -- while panic() may be more likely to succeed, it > is not gauranteed to. > >> 2.a. If CONFIG_SCHED_STACK_END_CHECK is enabled, the kernel already does panic() >> when STACK_END_MAGIC is corrupted. So we will _not_ break the safety policy if >> we do panic() in a similar situation in check_alloca(). > > Sure, I'm certainly happy with panic() here. Ok! >> 2.b. If CONFIG_SCHED_STACK_END_CHECK is disabled, the user has some real reasons >> not to do panic() when the kernel stack is corrupted. > > I believe that CONFIG_SCHED_STACK_END_CHECK is seen as a debug feature, > and hence people don't select it. I see CONFIG_SCHED_STACK_END_CHECK enabled by default in Ubuntu config... > I strongly doubt that people have > reasons to disable it other than not wanting the overhead associated > with debug features. I think it's not a question of performance here. There are cases when a system must live as long as possible (even partially corrupted) and must not die entirely. Oops is ok for those systems, but panic (full DoS) is not. > I think it is reasonable to panic() here even with CONFIG_VMAP_STACK > selected. It's too tough for CONFIG_VMAP_STACK on x86 - the system can proceed to live. Anyway, the check_alloca() code will not be shared between x86 and arm64, I've described the reasons in this thread. So I can have BUG() for CONFIG_VMAP_STACK on x86 and Laura can consistently use panic() on arm64. >> So we should not do it in check_alloca() as well, just use BUG() and >> hope for the best. > > Regardless of whether we BUG() or panic(), we're hoping for the best. > > Consistently using panic() here will keep things simpler, so any failure > reported will be easier to reason about, and easier to debug. Let me keep BUG() for !CONFIG_SCHED_STACK_END_CHECK. I beware of using panic() by default, let distro/user decide this. I remember very well how I was shouted at, when this one was merged: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ce6fa91b93630396ca220c33dd38ffc62686d499 Mark, I'm really grateful to you for such a nice code review! Alexander
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From: alex.popov@linux.com (Alexander Popov) To: linux-arm-kernel@lists.infradead.org Subject: [PATCH 2/2] arm64: Clear the stack Date: Mon, 14 May 2018 16:53:12 +0300 [thread overview] Message-ID: <c5bebf0b-9667-5725-2800-97c5a85635c4@linux.com> (raw) In-Reply-To: <20180514100639.v3erlzbuv2e4awfh@lakrids.cambridge.arm.com> On 14.05.2018 13:06, Mark Rutland wrote: > On Mon, May 14, 2018 at 12:35:25PM +0300, Alexander Popov wrote: >> On 14.05.2018 08:15, Mark Rutland wrote: >>> On Sun, May 13, 2018 at 11:40:07AM +0300, Alexander Popov wrote: >>>> So what would you think if I do the following in check_alloca(): >>>> >>>> if (size >= stack_left) { >>>> #if !defined(CONFIG_VMAP_STACK) && defined(CONFIG_SCHED_STACK_END_CHECK) >>>> panic("alloca over the kernel stack boundary\n"); >>>> #else >>>> BUG(); >>>> #endif >>> >>> Given this is already out-of-line, how about we always use panic(), regardless >>> of VMAP_STACK and SCHED_STACK_END_CHECK? i.e. just >>> >>> if (unlikely(size >= stack_left)) >>> panic("alloca over the kernel stack boundary"); >>> >>> If we have VMAP_STACK selected, and overflow during the panic, it's the same as >>> if we overflowed during the BUG(). It's likely that panic() will use less stack >>> space than BUG(), and the compiler can put the call in a slow path that >>> shouldn't affect most calls, so in all cases it's likely preferable. >> >> I'm sure that maintainers and Linus will strongly dislike my patch if I always >> use panic() here. panic() kills the whole kernel and we shouldn't use it when we >> can safely continue to work. >> >> Let me describe my logic. So let's have size >= stack_left on a thread stack. >> >> 1. If CONFIG_VMAP_STACK is enabled, we can safely use BUG(). Even if BUG() >> handling overflows the thread stack into the guard page, handle_stack_overflow() >> is called and the neighbour memory is not corrupted. The kernel can proceed to live. > > On arm64 with CONFIG_VMAP_STACK, a stack overflow will result in a > panic(). My understanding was that the same is true on x86. No, x86 CONFIG_VMAP_STACK only kills the offending process. I see it on my deep recursion test, the kernel continues to live. handle_stack_overflow() in arch/x86/kernel/traps.c calls die(). >> 2. If CONFIG_VMAP_STACK is disabled, BUG() handling can corrupt the neighbour >> kernel memory and cause the undefined behaviour of the whole kernel. I see it on >> my lkdtm test. That is a cogent reason for panic(). > > In this case, panic() can also corrupt the neighbour stack, and could > also fail. > > When CONFIG_VMAP_STACK is not selected, a stack overflow simply cannot > be handled reliably -- while panic() may be more likely to succeed, it > is not gauranteed to. > >> 2.a. If CONFIG_SCHED_STACK_END_CHECK is enabled, the kernel already does panic() >> when STACK_END_MAGIC is corrupted. So we will _not_ break the safety policy if >> we do panic() in a similar situation in check_alloca(). > > Sure, I'm certainly happy with panic() here. Ok! >> 2.b. If CONFIG_SCHED_STACK_END_CHECK is disabled, the user has some real reasons >> not to do panic() when the kernel stack is corrupted. > > I believe that CONFIG_SCHED_STACK_END_CHECK is seen as a debug feature, > and hence people don't select it. I see CONFIG_SCHED_STACK_END_CHECK enabled by default in Ubuntu config... > I strongly doubt that people have > reasons to disable it other than not wanting the overhead associated > with debug features. I think it's not a question of performance here. There are cases when a system must live as long as possible (even partially corrupted) and must not die entirely. Oops is ok for those systems, but panic (full DoS) is not. > I think it is reasonable to panic() here even with CONFIG_VMAP_STACK > selected. It's too tough for CONFIG_VMAP_STACK on x86 - the system can proceed to live. Anyway, the check_alloca() code will not be shared between x86 and arm64, I've described the reasons in this thread. So I can have BUG() for CONFIG_VMAP_STACK on x86 and Laura can consistently use panic() on arm64. >> So we should not do it in check_alloca() as well, just use BUG() and >> hope for the best. > > Regardless of whether we BUG() or panic(), we're hoping for the best. > > Consistently using panic() here will keep things simpler, so any failure > reported will be easier to reason about, and easier to debug. Let me keep BUG() for !CONFIG_SCHED_STACK_END_CHECK. I beware of using panic() by default, let distro/user decide this. I remember very well how I was shouted at, when this one was merged: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ce6fa91b93630396ca220c33dd38ffc62686d499 Mark, I'm really grateful to you for such a nice code review! Alexander
next prev parent reply other threads:[~2018-05-14 13:53 UTC|newest] Thread overview: 86+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-04-06 14:22 [PATCH v11 0/6] Introduce the STACKLEAK feature and a test for it Alexander Popov 2018-04-06 14:22 ` [PATCH v11 1/6] gcc-plugins: Clean up the cgraph_create_edge* macros Alexander Popov 2018-04-06 14:22 ` [PATCH v11 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls Alexander Popov 2018-04-16 18:29 ` Kees Cook 2018-04-18 18:33 ` Laura Abbott 2018-04-18 18:50 ` Dave Hansen 2018-04-24 1:03 ` Kees Cook 2018-04-24 4:23 ` Dave Hansen 2018-04-30 23:48 ` Kees Cook 2018-05-02 8:42 ` Thomas Gleixner 2018-05-02 12:38 ` Kees Cook 2018-05-02 12:39 ` Thomas Gleixner 2018-05-02 12:51 ` Kees Cook 2018-05-02 21:02 ` Kees Cook 2018-05-06 10:04 ` Thomas Gleixner 2018-04-06 14:22 ` [PATCH v11 3/6] gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack Alexander Popov 2018-04-06 14:22 ` [PATCH v11 4/6] lkdtm: Add a test for STACKLEAK Alexander Popov 2018-04-06 14:22 ` [PATCH v11 5/6] fs/proc: Show STACKLEAK metrics in the /proc file system Alexander Popov 2018-04-06 14:22 ` [PATCH v11 6/6] doc: self-protection: Add information about STACKLEAK feature Alexander Popov 2018-05-02 20:33 ` [PATCH 0/2] Stackleak for arm64 Laura Abbott 2018-05-02 20:33 ` Laura Abbott 2018-05-02 20:33 ` [PATCH 1/2] stackleak: Update " Laura Abbott 2018-05-02 20:33 ` Laura Abbott 2018-05-02 20:33 ` [PATCH 2/2] arm64: Clear the stack Laura Abbott 2018-05-02 20:33 ` Laura Abbott 2018-05-02 21:31 ` Kees Cook 2018-05-02 21:31 ` Kees Cook 2018-05-02 23:07 ` Laura Abbott 2018-05-02 23:07 ` Laura Abbott 2018-05-02 23:37 ` Kees Cook 2018-05-02 23:37 ` Kees Cook 2018-05-03 16:05 ` Alexander Popov 2018-05-03 16:05 ` Alexander Popov 2018-05-03 16:45 ` Kees Cook 2018-05-03 16:45 ` Kees Cook 2018-05-03 7:19 ` Mark Rutland 2018-05-03 7:19 ` Mark Rutland 2018-05-03 11:37 ` Ard Biesheuvel 2018-05-03 11:37 ` Ard Biesheuvel 2018-05-03 17:33 ` Alexander Popov 2018-05-03 17:33 ` Alexander Popov 2018-05-03 19:09 ` Laura Abbott 2018-05-03 19:09 ` Laura Abbott 2018-05-04 8:30 ` Alexander Popov 2018-05-04 8:30 ` Alexander Popov 2018-05-04 11:09 ` Mark Rutland 2018-05-04 11:09 ` Mark Rutland 2018-05-06 8:22 ` Alexander Popov 2018-05-06 8:22 ` Alexander Popov 2018-05-11 15:50 ` Alexander Popov 2018-05-11 15:50 ` Alexander Popov 2018-05-11 16:13 ` Mark Rutland 2018-05-11 16:13 ` Mark Rutland 2018-05-13 8:40 ` Alexander Popov 2018-05-13 8:40 ` Alexander Popov 2018-05-14 5:15 ` Mark Rutland 2018-05-14 5:15 ` Mark Rutland 2018-05-14 9:35 ` Alexander Popov 2018-05-14 9:35 ` Alexander Popov 2018-05-14 10:06 ` Mark Rutland 2018-05-14 10:06 ` Mark Rutland 2018-05-14 13:53 ` Alexander Popov [this message] 2018-05-14 13:53 ` Alexander Popov 2018-05-14 14:07 ` Mark Rutland 2018-05-14 14:07 ` Mark Rutland 2018-05-03 19:00 ` Laura Abbott 2018-05-03 19:00 ` Laura Abbott 2018-05-04 11:16 ` Mark Rutland 2018-05-04 11:16 ` Mark Rutland 2018-05-14 18:55 ` [PATCH v11 0/6] Introduce the STACKLEAK feature and a test for it Laura Abbott -- strict thread matches above, loose matches on Subject: below -- 2018-07-18 21:10 [PATCH 0/2] Stackleak for arm64 Laura Abbott 2018-07-18 21:10 ` [PATCH 2/2] arm64: Clear the stack Laura Abbott 2018-07-18 21:10 ` Laura Abbott 2018-07-19 2:20 ` Kees Cook 2018-07-19 2:20 ` Kees Cook 2018-07-19 10:41 ` Alexander Popov 2018-07-19 10:41 ` Alexander Popov 2018-07-19 11:41 ` Mark Rutland 2018-07-19 11:41 ` Mark Rutland 2018-02-21 1:13 [PATCH 0/2] Stackleak for arm64 Laura Abbott 2018-02-21 1:13 ` [PATCH 2/2] arm64: Clear the stack Laura Abbott 2018-02-21 1:13 ` Laura Abbott 2018-02-21 15:38 ` Mark Rutland 2018-02-21 15:38 ` Mark Rutland 2018-02-21 23:53 ` Laura Abbott 2018-02-21 23:53 ` Laura Abbott 2018-02-22 1:35 ` Laura Abbott 2018-02-22 1:35 ` Laura Abbott
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