Messages in this thread Patch in this message | | | Date | Tue, 26 Apr 2022 12:15:40 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH 0/8] stackleak: fixes and rework |
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On Tue, Apr 26, 2022 at 11:37:47AM +0100, Mark Rutland wrote: > 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...
That offset above seems to be due to the earlier logic in direct_entry(), which I guess is running out-of-line. With that hacked to:
---------------- diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c index e2228b6fc09bb..53f3027e8202d 100644 --- a/drivers/misc/lkdtm/core.c +++ b/drivers/misc/lkdtm/core.c @@ -378,8 +378,9 @@ static ssize_t direct_entry(struct file *f, const char __user *user_buf, size_t count, loff_t *off) { const struct crashtype *crashtype; - char *buf; + char *buf = "STACKLEAK_ERASING"; +#if 0 if (count >= PAGE_SIZE) return -EINVAL; if (count < 1) @@ -395,13 +396,17 @@ static ssize_t direct_entry(struct file *f, const char __user *user_buf, /* NULL-terminate and remove enter */ buf[count] = '\0'; strim(buf); +#endif crashtype = find_crashtype(buf); + +#if 0 free_page((unsigned long) buf); if (!crashtype) return -EINVAL; +#endif - pr_info("Performing direct entry %s\n", crashtype->name); + // pr_info("Performing direct entry %s\n", crashtype->name); lkdtm_do_action(crashtype); *off += count; ---------------- ... the SP check doesn't fail, but I still see intermittent bad value failures. Those might be due to interrupt frames.
Thanks, Mark.
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