Re: [PATCH 2/5] powerpc: Allow 4096 bytes of stack expansion for the signal frame
From: Daniel Axtens <hidden>
Date: 2020-07-23 13:35:45
Also in:
linux-arch, lkml
Hi Michael, Unfortunately, this patch doesn't completely solve the problem. Trying the original reproducer, I'm still able to trigger the crash even with this patch, although not 100% of the time. (If I turn ASLR off outside of tmux it reliably crashes, if I turn ASLR off _inside_ of tmux it reliably succeeds; all of this is on a serial console.) ./foo 1241000 & sleep 1; killall -USR1 foo; echo ok If I add some debugging information, I see that I'm getting address + 4096 = 7fffffed0fa0 gpr1 = 7fffffed1020 So address + 4096 is 0x80 bytes below the 4k window. I haven't been able to figure out why, gdb gives me a NIP in __kernel_sigtramp_rt64 but I don't know what to make of that. Kind regards, Daniel P.S. I don't know what your policy on linking to kernel bugzilla is, but if you want: Link: https://bugzilla.kernel.org/show_bug.cgi?id=205183
quoted hunk ↗ jump to hunk
Reported-by: Tom Lane <redacted> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> --- arch/powerpc/mm/fault.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c index 641fc5f3d7dd..ed01329dd12b 100644 --- a/arch/powerpc/mm/fault.c +++ b/arch/powerpc/mm/fault.c@@ -274,7 +274,7 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address, /* * N.B. The POWER/Open ABI allows programs to access up to * 288 bytes below the stack pointer. - * The kernel signal delivery code writes up to about 1.5kB + * The kernel signal delivery code writes up to 4KB * below the stack pointer (r1) before decrementing it. * The exec code can write slightly over 640kB to the stack * before setting the user r1. Thus we allow the stack to@@ -299,7 +299,7 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address, * between the last mapped region and the stack will * expand the stack rather than segfaulting. */ - if (address + 2048 >= uregs->gpr[1]) + if (address + 4096 >= uregs->gpr[1]) return false; if ((flags & FAULT_FLAG_WRITE) && (flags & FAULT_FLAG_USER) &&-- 2.25.1