From mboxrd@z Thu Jan 1 00:00:00 1970 MIME-Version: 1.0 Sender: keescook@google.com In-Reply-To: <20180326162158.GH16308@e103592.cambridge.arm.com> References: <1515636190-24061-1-git-send-email-keescook@chromium.org> <1515636190-24061-34-git-send-email-keescook@chromium.org> <20180115122458.GI12608@e103592.cambridge.arm.com> <20180326162158.GH16308@e103592.cambridge.arm.com> From: Kees Cook Date: Mon, 26 Mar 2018 10:41:09 -0700 Message-ID: Subject: Re: [PATCH 33/38] arm64: Implement thread_struct whitelist for hardened usercopy Content-Type: text/plain; charset="UTF-8" To: Dave Martin Cc: Mark Rutland , "kernel-hardening@lists.openwall.com" , Catalin Marinas , Will Deacon , "linux-arm-kernel@lists.infradead.org" List-ID: On Mon, Mar 26, 2018 at 9:22 AM, Dave Martin wrote: > [Dropped most of the original Cc list, since most people are unlikely to > care about this thread archaeology.] > > On Mon, Jan 15, 2018 at 12:06:17PM -0800, Kees Cook wrote: >> On Mon, Jan 15, 2018 at 4:24 AM, Dave P Martin wrote: >> > On Thu, Jan 11, 2018 at 02:03:05AM +0000, Kees Cook wrote: >> >> This whitelists the FPU register state portion of the thread_struct for >> >> copying to userspace, instead of the default entire structure. >> >> >> >> Cc: Catalin Marinas >> >> Cc: Will Deacon >> >> Cc: Christian Borntraeger >> >> Cc: Ingo Molnar >> >> Cc: James Morse >> >> Cc: "Peter Zijlstra (Intel)" >> >> Cc: Dave Martin >> >> Cc: zijun_hu >> >> Cc: linux-arm-kernel@lists.infradead.org >> >> Signed-off-by: Kees Cook >> >> --- >> >> arch/arm64/Kconfig | 1 + >> >> arch/arm64/include/asm/processor.h | 8 ++++++++ >> >> 2 files changed, 9 insertions(+) >> >> >> >> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig >> >> index a93339f5178f..c84477e6a884 100644 >> >> --- a/arch/arm64/Kconfig >> >> +++ b/arch/arm64/Kconfig >> >> @@ -90,6 +90,7 @@ config ARM64 >> >> select HAVE_ARCH_MMAP_RND_BITS >> >> select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT >> >> select HAVE_ARCH_SECCOMP_FILTER >> >> + select HAVE_ARCH_THREAD_STRUCT_WHITELIST >> >> select HAVE_ARCH_TRACEHOOK >> >> select HAVE_ARCH_TRANSPARENT_HUGEPAGE >> >> select HAVE_ARCH_VMAP_STACK >> >> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h >> >> index 023cacb946c3..e58a5864ec89 100644 >> >> --- a/arch/arm64/include/asm/processor.h >> >> +++ b/arch/arm64/include/asm/processor.h >> >> @@ -113,6 +113,14 @@ struct thread_struct { >> >> struct debug_info debug; /* debugging */ >> >> }; >> >> >> >> +/* Whitelist the fpsimd_state for copying to userspace. */ >> >> +static inline void arch_thread_struct_whitelist(unsigned long *offset, >> >> + unsigned long *size) >> >> +{ >> >> + *offset = offsetof(struct thread_struct, fpsimd_state); >> >> + *size = sizeof(struct fpsimd_state); >> > >> > This should be fpsimd_state.user_fpsimd (fpsimd_state.cpu is important >> > for correctly context switching and not supposed to be user-accessible. >> > A user copy that encompasses that is definitely a bug). >> >> So, I actually spent some more time looking at this due to the >> comments from rmk on arm32, and I don't think any whitelist is needed >> here at all. (i.e. it can be *offset = *size = 0) This is because all >> the usercopying I could find uses static sizes or bounce buffers, both >> of which bypass the dynamic-size hardened usercopy checks. >> >> I've been running some arm64 builds now with this change, and I >> haven't tripped over any problems yet... > > Hmmm, it looks like we may be hitting this with user_regset_copyout() > when reading the fp regs via ptrace. This is maybe not surprising, Did you get one of the WARNs for it? > since the size comes from userspace for PTRACE_{GET,SET}REGSET. > Also, while we copy into a bounce buffer for SETREGSET here, we do copy > straight out of task_struct for GETREGSET here. Hm, yeah, > This suggests we should have: > > *offset = offsetof(struct thread_struct, fpsimd_state); > *size = sizeof(struct user_fpsimd_state); This is what I had originally for arm64, but when I tried exercising this code more recently, it didn't need the whitelist. It really looks like I forgot what I had tested the first time, though. :P > Thoughts? Seems like it would be tripped by: static int __fpr_get(struct task_struct *target, const struct user_regset *regset, unsigned int pos, unsigned int count, void *kbuf, void __user *ubuf, unsigned int start_pos) { struct user_fpsimd_state *uregs; sve_sync_to_fpsimd(target); uregs = &target->thread.fpsimd_state.user_fpsimd; return user_regset_copyout(&pos, &count, &kbuf, &ubuf, uregs, start_pos, start_pos + sizeof(*uregs)); } And similarly __fpr_set(), compat_vfp_get(), compat_vfp_set(), sve_get(), and sve_set(), ? > I'm making some assumptions about how the usercopy hardening works. I think you're right -- I just tricked myself after looking at arm32. -Kees -- Kees Cook Pixel Security