[PATCH 1/3] arm64/ptrace: don't clobber task registers on syscall entry/exit traps

Andrei Vagin
2021-02-01T19:42:54Z

ip/r12 for AArch32 and x7 for AArch64 is used to indicate whether or not
the stop has been signalled from syscall entry or syscall exit. This
means that:

- Any writes by the tracer to this register during the stop are
ignored/discarded.

- The actual value of the register is not available during the stop,
so the tracer cannot save it and restore it later.

Right now, these registers are clobbered in tracehook_report_syscall.
This change moves the logic to gpr_get and compat_gpr_get where
registers are copied into a user-space buffer.

This will allow to change these registers and to introduce a new
ptrace option to get the full set of registers.

Signed-off-by: Andrei Vagin <avagin@gmail.com>
---
arch/arm64/include/asm/ptrace.h | 5 ++
arch/arm64/kernel/ptrace.c | 104 ++++++++++++++++++++------------
2 files changed, 69 insertions(+), 40 deletions(-)

diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index e58bca832dff..0a9552b4f61e 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -170,6 +170,11 @@ static inline unsigned long pstate_to_compat_psr(const unsigned long pstate)
return psr;
}

+enum ptrace_syscall_dir {
+ PTRACE_SYSCALL_ENTER = 0,
+ PTRACE_SYSCALL_EXIT,
+};
+
/*
* This struct defines the way the registers are stored on the stack during an
* exception. Note that sizeof(struct pt_regs) has to be a multiple of 16 (for
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 8ac487c84e37..39da03104528 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -40,6 +40,7 @@
#include <asm/syscall.h>
#include <asm/traps.h>
#include <asm/system_misc.h>
+#include <asm/ptrace.h>

#define CREATE_TRACE_POINTS
#include <trace/events/syscalls.h>
@@ -561,7 +562,31 @@ static int gpr_get(struct task_struct *target,
struct membuf to)
{
struct user_pt_regs *uregs = &task_pt_regs(target)->user_regs;
- return membuf_write(&to, uregs, sizeof(*uregs));
+ unsigned long saved_reg;
+ int ret;
+
+ /*
+ * We have some ABI weirdness here in the way that we handle syscall
+ * exit stops because we indicate whether or not the stop has been
+ * signalled from syscall entry or syscall exit by clobbering the general
+ * purpose register x7.
+ */
+ saved_reg = uregs->regs[7];
+
+ switch (target->ptrace_message) {
+ case PTRACE_EVENTMSG_SYSCALL_ENTRY:
+ uregs->regs[7] = PTRACE_SYSCALL_ENTER;
+ break;
+ case PTRACE_EVENTMSG_SYSCALL_EXIT:
+ uregs->regs[7] = PTRACE_SYSCALL_EXIT;
+ break;
+ }
+
+ ret = membuf_write(&to, uregs, sizeof(*uregs));
+
+ uregs->regs[7] = saved_reg;
+
+ return ret;
}

static int gpr_set(struct task_struct *target, const struct user_regset *regset,
@@ -575,6 +600,17 @@ static int gpr_set(struct task_struct *target, const struct user_regset *regset,
if (ret)
return ret;

+ /*
+ * Historically, x7 can't be changed if the stop has been signalled
+ * from syscall-enter of syscall-exit.
+ */
+ switch (target->ptrace_message) {
+ case PTRACE_EVENTMSG_SYSCALL_ENTRY:
+ case PTRACE_EVENTMSG_SYSCALL_EXIT:
+ newregs.regs[7] = task_pt_regs(target)->regs[7];
+ break;
+ }
+
if (!valid_user_regs(&newregs, target))
return -EINVAL;

@@ -1206,6 +1242,20 @@ static inline compat_ulong_t compat_get_user_reg(struct task_struct *task, int i
struct pt_regs *regs = task_pt_regs(task);

switch (idx) {
+ case 12:
+ /*
+ * We have some ABI weirdness here in the way that we handle
+ * syscall exit stops because we indicate whether or not the
+ * stop has been signalled from syscall entry or syscall exit
+ * by clobbering the general purpose register r12.
+ */
+ switch (task->ptrace_message) {
+ case PTRACE_EVENTMSG_SYSCALL_ENTRY:
+ return PTRACE_SYSCALL_ENTER;
+ case PTRACE_EVENTMSG_SYSCALL_EXIT:
+ return PTRACE_SYSCALL_EXIT;
+ }
+ return regs->regs[idx];
case 15:
return regs->pc;
case 16:
@@ -1282,6 +1332,17 @@ static int compat_gpr_set(struct task_struct *target,

}

+ /*
+ * Historically, x12 can't be changed if the stop has been signalled
+ * from syscall-enter of syscall-exit.
+ */
+ switch (target->ptrace_message) {
+ case PTRACE_EVENTMSG_SYSCALL_ENTRY:
+ case PTRACE_EVENTMSG_SYSCALL_EXIT:
+ newregs.regs[12] = task_pt_regs(target)->regs[12];
+ break;
+ }
+
if (valid_user_regs(&newregs.user_regs, target))
*task_pt_regs(target) = newregs;
else
@@ -1740,53 +1801,16 @@ long arch_ptrace(struct task_struct *child, long request,
return ptrace_request(child, request, addr, data);
}

-enum ptrace_syscall_dir {
- PTRACE_SYSCALL_ENTER = 0,
- PTRACE_SYSCALL_EXIT,
-};
-
static void tracehook_report_syscall(struct pt_regs *regs,
enum ptrace_syscall_dir dir)
{
- int regno;
- unsigned long saved_reg;
-
- /*
- * We have some ABI weirdness here in the way that we handle syscall
- * exit stops because we indicate whether or not the stop has been
- * signalled from syscall entry or syscall exit by clobbering a general
- * purpose register (ip/r12 for AArch32, x7 for AArch64) in the tracee
- * and restoring its old value after the stop. This means that:
- *
- * - Any writes by the tracer to this register during the stop are
- * ignored/discarded.
- *
- * - The actual value of the register is not available during the stop,
- * so the tracer cannot save it and restore it later.
- *
- * - Syscall stops behave differently to seccomp and pseudo-step traps
- * (the latter do not nobble any registers).
- */
- regno = (is_compat_task() ? 12 : 7);
- saved_reg = regs->regs[regno];
- regs->regs[regno] = dir;
-
if (dir == PTRACE_SYSCALL_ENTER) {
if (tracehook_report_syscall_entry(regs))
forget_syscall(regs);
- regs->regs[regno] = saved_reg;
- } else if (!test_thread_flag(TIF_SINGLESTEP)) {
- tracehook_report_syscall_exit(regs, 0);
- regs->regs[regno] = saved_reg;
} else {
- regs->regs[regno] = saved_reg;
+ int singlestep = test_thread_flag(TIF_SINGLESTEP);

- /*
- * Signal a pseudo-step exception since we are stepping but
- * tracer modifications to the registers may have rewound the
- * state machine.
- */
- tracehook_report_syscall_exit(regs, 1);
+ tracehook_report_syscall_exit(regs, singlestep);
}
}

--
2.29.2