Messages in this thread | | | Date | Wed, 4 May 2022 17:41:32 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH v2 01/13] arm64: stackleak: fix current_top_of_stack() |
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On Wed, Apr 27, 2022 at 06:31:16PM +0100, Mark Rutland wrote: > Due to some historical confusion, arm64's current_top_of_stack() isn't > what the stackleak code expects. This could in theory result in a number > of problems, and practically results in an unnecessary performance hit. > We can avoid this by aligning the arm64 implementation with the x86 > implementation. > > The arm64 implementation of current_top_of_stack() was added > specifically for stackleak in commit: > > 0b3e336601b82c6a ("arm64: Add support for STACKLEAK gcc plugin") > > This was intended to be equivalent to the x86 implementation, but the > implementation, semantics, and performance characteristics differ > wildly: > > * On x86, current_top_of_stack() returns the top of the current task's > task stack, regardless of which stack is in active use. > > The implementation accesses a percpu variable which the x86 entry code > maintains, and returns the location immediately above the pt_regs on > the task stack (above which x86 has some padding). > > * On arm64 current_top_of_stack() returns the top of the stack in active > use (i.e. the one which is currently being used). > > The implementation checks the SP against a number of > potentially-accessible stacks, and will BUG() if no stack is found. > > The core stackleak_erase() code determines the upper bound of stack to > erase with: > > | if (on_thread_stack()) > | boundary = current_stack_pointer; > | else > | boundary = current_top_of_stack(); > > On arm64 stackleak_erase() is always called on a task stack, and > on_thread_stack() should always be true. On x86, stackleak_erase() is > mostly called on a trampoline stack, and is sometimes called on a task > stack. > > Currently, this results in a lot of unnecessary code being generated for > arm64 for the impossible !on_thread_stack() case. Some of this is > inlined, bloating stackleak_erase(), while portions of this are left > out-of-line and permitted to be instrumented (which would be a > functional problem if that code were reachable). > > As a first step towards improving this, this patch aligns arm64's > implementation of current_top_of_stack() with x86's, always returning > the top of the current task's stack. With GCC 11.1.0 this results in the > bulk of the unnecessary code being removed, including all of the > out-of-line instrumentable code. > > While I don't believe there's a functional problem in practice I've > marked this as a fix since the semantic was clearly wrong, the fix > itself is simple, and other code might rely upon this in future. > > Fixes: 0b3e336601b82c6a ("arm64: Add support for STACKLEAK gcc plugin") > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > Cc: Alexander Popov <alex.popov@linux.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Andy Lutomirski <luto@kernel.org> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: Kees Cook <keescook@chromium.org> > Cc: Will Deacon <will@kernel.org>
I thought this was queued already but I couldn't find it in -next. So:
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
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