[PATCH 33/38] arm64: Implement thread_struct whitelist for hardened usercopy

[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 [off-list ref] wrote:

quoted

On Thu, Jan 11, 2018 at 02:03:05AM +0000, Kees Cook wrote:

quoted

This whitelists the FPU register state portion of the thread_struct for
copying to userspace, instead of the default entire structure.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <redacted>
Cc: Christian Borntraeger <redacted>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: zijun_hu <redacted>
Cc: linux-arm-kernel at lists.infradead.org
Signed-off-by: Kees Cook <redacted>
---
 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,
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.

This suggests we should have:

	*offset = offsetof(struct thread_struct, fpsimd_state);
	*size = sizeof(struct user_fpsimd_state);

Thoughts?

I'm making some assumptions about how the usercopy hardening works.

Cheers
---Dave