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x86/vsyscall: Document odd SIGSEGV error code for vsyscalls

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Even if vsyscall=none, user page faults on the vsyscall page are reported
as though the PROT bit in the error code was set.  Add a comment explaining
why this is probably okay and display the value in the test case.

While at it, explain why the behavior is correct with respect to PKRU.

Modify also the selftest to print the odd error code so that there is a
way to demonstrate the odd behaviour.

If anyone really cares about more accurate emulation, the behaviour could
be changed. But that needs a real good justification.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/75c91855fd850649ace162eec5495a1354221aaa.1561610354.git.luto@kernel.org
This commit is contained in:
Andy Lutomirski
2019-06-26 21:45:05 -07:00
committed by Thomas Gleixner
parent 918ce32509
commit e0a446ce39
2 changed files with 15 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

9
tools/testing/selftests/x86/test_vsyscall.c
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@@ -183,9 +183,13 @@ static inline long sys_getcpu(unsigned * cpu, unsigned * node,
}
static jmp_buf jmpbuf;
static volatile unsigned long segv_err;
static void sigsegv(int sig, siginfo_t *info, void *ctx_void)
{
ucontext_t *ctx = (ucontext_t *)ctx_void;
segv_err = ctx->uc_mcontext.gregs[REG_ERR];
siglongjmp(jmpbuf, 1);
}
@@ -416,8 +420,11 @@ static int test_vsys_r(void)
} else if (!can_read && should_read_vsyscall) {
printf("[FAIL]\tWe don't have read access, but we should\n");
return 1;
} else if (can_read) {
printf("[OK]\tWe have read access\n");
} else {
printf("[OK]\tgot expected result\n");
printf("[OK]\tWe do not have read access: #PF(0x%lx)\n",
segv_err);
}
#endif