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selftests/x86/fsgsbase: Test GS selector on ptracer-induced GS base write

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The test validates that the selector is not changed when a ptracer writes
the ptracee's GS base.

Originally-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20200528201402.1708239-16-sashal@kernel.org
This commit is contained in:
Chang S. Bae 2020-05-28 16:14:01 -04:00 committed by Thomas Gleixner
parent 82c0c7d24c
commit 291fd83569
1 changed files with 15 additions and 6 deletions
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21
tools/testing/selftests/x86/fsgsbase.c
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@ -465,7 +465,7 @@ static void test_ptrace_write_gsbase(void)
wait(&status);
if (WSTOPSIG(status) == SIGTRAP) {
unsigned long gs, base;
unsigned long gs;
unsigned long gs_offset = USER_REGS_OFFSET(gs);
unsigned long base_offset = USER_REGS_OFFSET(gs_base);
@ -481,7 +481,6 @@ static void test_ptrace_write_gsbase(void)
err(1, "PTRACE_POKEUSER");
gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);
base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);
/*
* In a non-FSGSBASE system, the nonzero selector will load
@ -489,11 +488,21 @@ static void test_ptrace_write_gsbase(void)
* selector value is changed or not by the GSBASE write in
* a ptracer.
*/
if (gs == 0 && base == 0xFF) {
printf("[OK]\tGS was reset as expected\n");
} else {
if (gs != *shared_scratch) {
nerrs++;
printf("[FAIL]\tGS=0x%lx, GSBASE=0x%lx (should be 0, 0xFF)\n", gs, base);
printf("[FAIL]\tGS changed to %lx\n", gs);
/*
* On older kernels, poking a nonzero value into the
* base would zero the selector. On newer kernels,
* this behavior has changed -- poking the base
* changes only the base and, if FSGSBASE is not
* available, this may not effect.
*/
if (gs == 0)
printf("\tNote: this is expected behavior on older kernels.\n");
} else {
printf("[OK]\tGS remained 0x%hx\n", *shared_scratch);
}
}