[Crash-utility] arm64: bug report: segfault

[Andrew Jones]

Hi Dave,

When crash gets a prstatus for an AArch32 (compat) user mode stack frame,
but assumes it can look at prstatus->sp for the stack pointer, and
prstatus->sp has a stale AArch64 kernel address in it, then crash
segfaults.

This is because the stack pointer isn't a user stack address, and thus
crash expects the stack to include at least two frames, which would mean
fp is non-zero, but in this case it's not[1], and that leads to the
calculation of a bad stack pointer (see arm64.c:1209), which then gets
used as an offset into the stack buffer (see arm64.c:1001), overflowing
it.

The patch[2] I just sent resolves this issue for me, but only because
it no longer tries to use prstatus->sp. We should probably still look
into fixing this in another way, such as making sure fp is non-zero, as
dumps can have all sorts of corruption breaking our assumptions.

Thanks,
drew

[1] The dump was captured with QEMU, which doesn't require a real crash,
    i.e. panic() being called in the guest kernel, thus cpus can actually
    be in user mode, rather than in the 64-bit cpu-stop IPI handler, or
    other crashing kernel code.
[2] https://www.redhat.com/archives/crash-utility/2015-December/msg00024.html