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selftests/x86/mov_ss_trap: Fix the SYSENTER test

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commit 8caa016bfc upstream.

For reasons that I haven't quite fully diagnosed, running
mov_ss_trap_32 on a 32-bit kernel results in an infinite loop in
userspace.  This appears to be because the hacky SYSENTER test
doesn't segfault as desired; instead it corrupts the program state
such that it infinite loops.

Fix it by explicitly clearing EBP before doing SYSENTER.  This will
give a more reliable segfault.

Fixes: 59c2a7226f ("x86/selftests: Add mov_to_ss test")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Andy Lutomirski
2019-11-20 12:59:13 -08:00
committed by Greg Kroah-Hartman
parent e74c494a20
commit 72e521b167
1 changed files with 2 additions and 1 deletions
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3
tools/testing/selftests/x86/mov_ss_trap.c
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@@ -257,7 +257,8 @@ int main()
err(1, "sigaltstack");
sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND | SA_ONSTACK);
nr = SYS_getpid;
asm volatile ("mov %[ss], %%ss; SYSENTER" : "+a" (nr)
/* Clear EBP first to make sure we segfault cleanly. */
asm volatile ("xorl %%ebp, %%ebp; mov %[ss], %%ss; SYSENTER" : "+a" (nr)
: [ss] "m" (ss) : "flags", "rcx"
#ifdef __x86_64__
, "r11"