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-rw-r--r--arch/x86/entry/entry_64.S5
-rw-r--r--arch/x86/include/asm/fsgsbase.h4
-rw-r--r--arch/x86/kernel/process_64.c8
-rw-r--r--tools/testing/selftests/x86/fsgsbase.c68
4 files changed, 77 insertions, 8 deletions
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index d977079a7d02..748c2db3c54e 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -842,8 +842,9 @@ SYM_CODE_START_LOCAL(paranoid_entry)
* retrieve and set the current CPUs kernel GSBASE. The stored value
* has to be restored in paranoid_exit unconditionally.
*
- * The MSR write ensures that no subsequent load is based on a
- * mispredicted GSBASE. No extra FENCE required.
+ * The unconditional write to GS base below ensures that no subsequent
+ * loads based on a mispredicted GS base can happen, therefore no LFENCE
+ * is needed here.
*/
SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
ret
diff --git a/arch/x86/include/asm/fsgsbase.h b/arch/x86/include/asm/fsgsbase.h
index d552646411a9..35cff5f2becf 100644
--- a/arch/x86/include/asm/fsgsbase.h
+++ b/arch/x86/include/asm/fsgsbase.h
@@ -57,7 +57,7 @@ static inline unsigned long x86_fsbase_read_cpu(void)
{
unsigned long fsbase;
- if (static_cpu_has(X86_FEATURE_FSGSBASE))
+ if (boot_cpu_has(X86_FEATURE_FSGSBASE))
fsbase = rdfsbase();
else
rdmsrl(MSR_FS_BASE, fsbase);
@@ -67,7 +67,7 @@ static inline unsigned long x86_fsbase_read_cpu(void)
static inline void x86_fsbase_write_cpu(unsigned long fsbase)
{
- if (static_cpu_has(X86_FEATURE_FSGSBASE))
+ if (boot_cpu_has(X86_FEATURE_FSGSBASE))
wrfsbase(fsbase);
else
wrmsrl(MSR_FS_BASE, fsbase);
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 9afefe325acb..df342bedea88 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -407,7 +407,7 @@ unsigned long x86_gsbase_read_cpu_inactive(void)
{
unsigned long gsbase;
- if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
+ if (boot_cpu_has(X86_FEATURE_FSGSBASE)) {
unsigned long flags;
local_irq_save(flags);
@@ -422,7 +422,7 @@ unsigned long x86_gsbase_read_cpu_inactive(void)
void x86_gsbase_write_cpu_inactive(unsigned long gsbase)
{
- if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
+ if (boot_cpu_has(X86_FEATURE_FSGSBASE)) {
unsigned long flags;
local_irq_save(flags);
@@ -439,7 +439,7 @@ unsigned long x86_fsbase_read_task(struct task_struct *task)
if (task == current)
fsbase = x86_fsbase_read_cpu();
- else if (static_cpu_has(X86_FEATURE_FSGSBASE) ||
+ else if (boot_cpu_has(X86_FEATURE_FSGSBASE) ||
(task->thread.fsindex == 0))
fsbase = task->thread.fsbase;
else
@@ -454,7 +454,7 @@ unsigned long x86_gsbase_read_task(struct task_struct *task)
if (task == current)
gsbase = x86_gsbase_read_cpu_inactive();
- else if (static_cpu_has(X86_FEATURE_FSGSBASE) ||
+ else if (boot_cpu_has(X86_FEATURE_FSGSBASE) ||
(task->thread.gsindex == 0))
gsbase = task->thread.gsbase;
else
diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
index 998319553523..7161cfc2e60b 100644
--- a/tools/testing/selftests/x86/fsgsbase.c
+++ b/tools/testing/selftests/x86/fsgsbase.c
@@ -443,6 +443,68 @@ static void test_unexpected_base(void)
#define USER_REGS_OFFSET(r) offsetof(struct user_regs_struct, r)
+static void test_ptrace_write_gs_read_base(void)
+{
+ int status;
+ pid_t child = fork();
+
+ if (child < 0)
+ err(1, "fork");
+
+ if (child == 0) {
+ printf("[RUN]\tPTRACE_POKE GS, read GSBASE back\n");
+
+ printf("[RUN]\tARCH_SET_GS to 1\n");
+ if (syscall(SYS_arch_prctl, ARCH_SET_GS, 1) != 0)
+ err(1, "ARCH_SET_GS");
+
+ if (ptrace(PTRACE_TRACEME, 0, NULL, NULL) != 0)
+ err(1, "PTRACE_TRACEME");
+
+ raise(SIGTRAP);
+ _exit(0);
+ }
+
+ wait(&status);
+
+ if (WSTOPSIG(status) == SIGTRAP) {
+ unsigned long base;
+ unsigned long gs_offset = USER_REGS_OFFSET(gs);
+ unsigned long base_offset = USER_REGS_OFFSET(gs_base);
+
+ /* Read the initial base. It should be 1. */
+ base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);
+ if (base == 1) {
+ printf("[OK]\tGSBASE started at 1\n");
+ } else {
+ nerrs++;
+ printf("[FAIL]\tGSBASE started at 0x%lx\n", base);
+ }
+
+ printf("[RUN]\tSet GS = 0x7, read GSBASE\n");
+
+ /* Poke an LDT selector into GS. */
+ if (ptrace(PTRACE_POKEUSER, child, gs_offset, 0x7) != 0)
+ err(1, "PTRACE_POKEUSER");
+
+ /* And read the base. */
+ base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);
+
+ if (base == 0 || base == 1) {
+ printf("[OK]\tGSBASE reads as 0x%lx with invalid GS\n", base);
+ } else {
+ nerrs++;
+ printf("[FAIL]\tGSBASE=0x%lx (should be 0 or 1)\n", base);
+ }
+ }
+
+ ptrace(PTRACE_CONT, child, NULL, NULL);
+
+ wait(&status);
+ if (!WIFEXITED(status))
+ printf("[WARN]\tChild didn't exit cleanly.\n");
+}
+
static void test_ptrace_write_gsbase(void)
{
int status;
@@ -517,6 +579,9 @@ static void test_ptrace_write_gsbase(void)
END:
ptrace(PTRACE_CONT, child, NULL, NULL);
+ wait(&status);
+ if (!WIFEXITED(status))
+ printf("[WARN]\tChild didn't exit cleanly.\n");
}
int main()
@@ -526,6 +591,9 @@ int main()
shared_scratch = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_SHARED, -1, 0);
+ /* Do these tests before we have an LDT. */
+ test_ptrace_write_gs_read_base();
+
/* Probe FSGSBASE */
sethandler(SIGILL, sigill, 0);
if (sigsetjmp(jmpbuf, 1) == 0) {