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selftests/x86/fsgsbase: Test ptracer-induced GSBASE write

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

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://lkml.kernel.org/r/1557309753-24073-3-git-send-email-chang.seok.bae@intel.com
This commit is contained in:
Chang S. Bae 2019-05-08 03:02:17 -07:00 committed by Thomas Gleixner
parent 48f5e52e91
commit 1b6858d5a2
1 changed files with 70 additions and 0 deletions
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70
tools/testing/selftests/x86/fsgsbase.c
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@ -23,6 +23,9 @@
#include <pthread.h>
#include <asm/ldt.h>
#include <sys/mman.h>
#include <stddef.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#ifndef __x86_64__
# error This test is 64-bit only
@ -367,6 +370,71 @@ static void test_unexpected_base(void)
}
}
#define USER_REGS_OFFSET(r) offsetof(struct user_regs_struct, r)
static void test_ptrace_write_gsbase(void)
{
int status;
pid_t child = fork();
if (child < 0)
err(1, "fork");
if (child == 0) {
printf("[RUN]\tPTRACE_POKE(), write GSBASE from ptracer\n");
/*
* Use the LDT setup and fetch the GSBASE from the LDT
* by switching to the (nonzero) selector (again)
*/
do_unexpected_base();
asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0x7));
if (ptrace(PTRACE_TRACEME, 0, NULL, NULL) != 0)
err(1, "PTRACE_TRACEME");
raise(SIGTRAP);
_exit(0);
}
wait(&status);
if (WSTOPSIG(status) == SIGTRAP) {
unsigned long gs;
unsigned long gs_offset = USER_REGS_OFFSET(gs);
unsigned long base_offset = USER_REGS_OFFSET(gs_base);
gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);
if (gs != 0x7) {
nerrs++;
printf("[FAIL]\tGS is not prepared with nonzero\n");
goto END;
}
if (ptrace(PTRACE_POKEUSER, child, base_offset, 0xFF) != 0)
err(1, "PTRACE_POKEUSER");
gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);
/*
* In a non-FSGSBASE system, the nonzero selector will load
* GSBASE (again). But what is tested here is whether the
* selector value is changed or not by the GSBASE write in
* a ptracer.
*/
if (gs != 0x7) {
nerrs++;
printf("[FAIL]\tGS changed to %lx\n", gs);
} else {
printf("[OK]\tGS remained 0x7\n");
}
}
END:
ptrace(PTRACE_CONT, child, NULL, NULL);
}
int main()
{
pthread_t thread;
@ -423,5 +491,7 @@ int main()
if (pthread_join(thread, NULL) != 0)
err(1, "pthread_join");
test_ptrace_write_gsbase();
return nerrs == 0 ? 0 : 1;
}