Messages in this thread | | | Date | Tue, 26 Apr 2022 11:37:47 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH 0/8] stackleak: fixes and rework |
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On Tue, Apr 26, 2022 at 11:10:52AM +0100, Mark Rutland wrote: > On Mon, Apr 25, 2022 at 03:54:00PM -0700, Kees Cook wrote: > > On Mon, Apr 25, 2022 at 12:55:55PM +0100, Mark Rutland wrote: > > > This series reworks the stackleak code. The first patch fixes some > > > latent issues on arm64, and the subsequent patches improve the code to > > > improve clarity and permit better code generation. > > > > This looks nice; thanks! I'll put this through build testing and get it > > applied shortly... > > Thanks! > > Patch 1 is liable to conflict with come other stacktrace bits that may go in > for v5.19, so it'd be good if either that could be queued as a fix for > v5.1-rc4, or we'll have to figure out how to deal with conflicts later. > > > > While the improvement is small, I think the improvement to clarity and > > > code generation is a win regardless. > > > > Agreed. I also want to manually inspect the resulting memory just to > > make sure things didn't accidentally regress. There's also an LKDTM test > > for basic functionality. > > I assume that's the STACKLEAK_ERASING test? > > I gave that a spin, but on arm64 that test is flaky even on baseline v5.18-rc1. > On x86_64 it seems consistent after 100s of runs. I'll go dig into that now.
I hacked in some debug, and it looks like the sp used in the test is far above the current lowest_sp. The test is slightly wrong since it grabs the address of a local variable rather than using current_stack_pointer, but the offset I see is much larger:
# echo STACKLEAK_ERASING > /sys/kernel/debug/provoke-crash/DIRECT [ 27.665221] lkdtm: Performing direct entry STACKLEAK_ERASING [ 27.665986] lkdtm: FAIL: lowest_stack 0xffff8000083a39e0 is lower than test sp 0xffff8000083a3c80 [ 27.667530] lkdtm: FAIL: the thread stack is NOT properly erased!
That's off by 0x2a0 (AKA 672) bytes, and it seems to be consistent from run to run.
I note that an interrupt occuring could cause similar (since on arm64 those are taken/triaged on the task stack before moving to the irq stack, and the irq regs alone will take 300+ bytes), but that doesn't seem to be the problem here given this is consistent, and it appears some prior function consumed a lot of stack.
I *think* the same irq problem would apply to x86, but maybe that initial triage happens on a trampoline stack.
I'll dig a bit more into the arm64 side...
Thanks, Mark.
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