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Subject[PATCH v13 16/16] selftests/x86/fsgsbase: Test ptracer-induced GS base write with FSGSBASE
Date
From: "Chang S. Bae" <chang.seok.bae@intel.com>

This validates that GS selector and base are independently preserved in
ptrace commands.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
tools/testing/selftests/x86/fsgsbase.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
index 950a48b2e366..9a4349813a30 100644
--- a/tools/testing/selftests/x86/fsgsbase.c
+++ b/tools/testing/selftests/x86/fsgsbase.c
@@ -465,7 +465,7 @@ static void test_ptrace_write_gsbase(void)
wait(&status);

if (WSTOPSIG(status) == SIGTRAP) {
- unsigned long gs;
+ unsigned long gs, base;
unsigned long gs_offset = USER_REGS_OFFSET(gs);
unsigned long base_offset = USER_REGS_OFFSET(gs_base);

@@ -481,6 +481,7 @@ 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
@@ -501,8 +502,14 @@ static void test_ptrace_write_gsbase(void)
*/
if (gs == 0)
printf("\tNote: this is expected behavior on older kernels.\n");
+ } else if (have_fsgsbase && (base != 0xFF)) {
+ nerrs++;
+ printf("[FAIL]\tGSBASE changed to %lx\n", base);
} else {
- printf("[OK]\tGS remained 0x%hx\n", *shared_scratch);
+ printf("[OK]\tGS remained 0x%hx", *shared_scratch);
+ if (have_fsgsbase)
+ printf(" and GSBASE changed to 0xFF");
+ printf("\n");
}
}

--
2.25.1
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