From: Bernhard M. Wiedemann <[email protected]>
syscall_exit_prepare() asks the exit assembly to reload all GPRs, CTR
and XER from pt_regs only if the thread flags contain any of the flags
in _TIF_SYSCALL_DOTRACE. With the new entry code based GENERIC_ENTRY the
flags are no longer set.
This breaks tracers that modify a stopped syscall, for example ones that
re-execute the syscall instruction by moving the NIP back by 4 at the
syscall-exit stop with the syscall number put back into r0: the sc then
runs with r0 = 0, which is __NR_restart_syscall. dettrace fails like
this with "No filter rule for system call: restart_syscall". A tracer
that writes r4 at the syscall-exit stop of an sc sees the tracee
continue with r4 = 0 instead; with a 7.1 kernel both work.
Check the equivalent syscall_work bits instead, and drop the now unused
_TIF_SYSCALL_DOTRACE.
Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
Cc: [email protected] # 7.2.x: c7585b8e99ad: powerpc: Don't drop
_TIF_RESTOREALL on syscall restart
Cc: [email protected] # 7.2.x
Assisted-by: LLM
Signed-off-by: Bernhard M. Wiedemann <[email protected]>
[ms: reword commit message]
Signed-off-by: Michal Suchánek <[email protected]>
---
A backport also needs commit c7585b8e99ad ("powerpc: Don't drop
_TIF_RESTOREALL on syscall restart"), without which the flag can still
be lost.
v2: rename PPC_SYSCALL_WORK_RESTOREALL to SYSCALL_WORK_RESTOREALL and
move it from interrupt.c to thread_info.h
v1:
https://lore.kernel.org/linuxppc-dev/4c1da7f31a6603455ec452960af529e4d77f5334.1791826411.git.msucha...@suse.de/
Testing: a small tracer (pokes r4 at the syscall-exit stop of an sc,
and replays a syscall by moving NIP back with a new r0) shows both
writes lost on a ppc64le and both working on 7.1.8.
------------[ cut here ]------------
// SPDX-License-Identifier: GPL-2.0
/*
* Does a register a tracer writes at the syscall-exit stop of an "sc"
* reach the tracee? Checks r4, and a replay: NIP moved back onto the sc
* with r0 set to another syscall number.
* Build: gcc -O2 -o ppc-exitregs ppc-exitregs.c (powerpc64 only)
*/
/* word offsets in struct pt_regs, arch/powerpc/include/uapi/asm/ptrace.h */
static long do_sc(long nr, long *r4io)
{
register long r0 __asm__("r0") = nr;
register long r3 __asm__("r3") = 0;
register long r4 __asm__("r4") = *r4io;
__asm__ volatile("sc"
: "+r"(r0), "+r"(r3), "+r"(r4)
:
: "r5", "r6", "r7", "r8", "r9", "r10", "r11", "r12",
"cr0", "ctr", "xer", "memory");
*r4io = r4;
return r3;
}
static long peek(pid_t pid, int w)
{
errno = 0;
long v = ptrace(PTRACE_PEEKUSER, pid, (void *)(w * sizeof(long)), 0);
if (errno) {
perror("PEEKUSER");
exit(2);
}
return v;
}
static void poke(pid_t pid, int w, long v)
{
if (ptrace(PTRACE_POKEUSER, pid, (void *)(w * sizeof(long)), (void
*)v)) {
perror("POKEUSER");
exit(2);
}
}
/* Resume to the next syscall stop, which must be one. */
static void next_stop(pid_t pid)
{
int st;
ptrace(PTRACE_SYSCALL, pid, 0, 0);
if (waitpid(pid, &st, 0) != pid || !WIFSTOPPED(st) ||
WSTOPSIG(st) != (SIGTRAP | 0x80)) {
fprintf(stderr, "unexpected wait status %#x\n", st);
exit(2);
}
}
int main(void)
{
pid_t pid = fork();
int st, fail = 0;
if (pid == 0) {
long r4 = 0x1111, r4a, ret;
ptrace(PTRACE_TRACEME, 0, 0, 0);
raise(SIGSTOP);
do_sc(__NR_getpid, &r4);
r4a = r4;
ret = do_sc(__NR_getpid, &r4);
/* bit 0: the tracer's r4 was lost; bit 1: the replay of
* the second getpid as getppid returned something else */
_exit((r4a != 0x2222) | (ret != getppid()) << 1);
}
waitpid(pid, &st, 0);
ptrace(PTRACE_SETOPTIONS, pid, 0, (void *)PTRACE_O_TRACESYSGOOD);
/* 1: write r4 at the exit stop of getpid */
next_stop(pid); /* entry */
next_stop(pid); /* exit */
poke(pid, W_R4, 0x2222);
next_stop(pid); /* entry of the second getpid */
/* 2: replay the second getpid as getppid */
next_stop(pid); /* exit */
poke(pid, W_NIP, peek(pid, W_NIP) - 4);
poke(pid, W_R0, __NR_getppid);
next_stop(pid); /* entry of the replay */
long nr = peek(pid, W_R0);
printf("replayed syscall arrives as nr %ld (%s)\n", nr,
nr == __NR_getppid ? "getppid, OK" :
nr == __NR_restart_syscall ? "restart_syscall, BUG" : "?");
if (nr != __NR_getppid) {
fail = 1;
poke(pid, W_R0, __NR_getppid); /* entry writes do work */
}
next_stop(pid); /* exit */
ptrace(PTRACE_CONT, pid, 0, 0);
waitpid(pid, &st, 0);
if (!WIFEXITED(st)) {
printf("child did not exit normally: %#x\n", st);
return 2;
}
printf("r4 written at the exit stop: %s\n",
WEXITSTATUS(st) & 1 ? "lost, BUG" : "kept, OK");
printf("replay result: %s\n",
WEXITSTATUS(st) & 2 ? "wrong" : "getppid(), OK");
return fail || WEXITSTATUS(st);
}
---
arch/powerpc/include/asm/thread_info.h | 15 ++++++++++++---
arch/powerpc/kernel/interrupt.c | 3 +--
2 files changed, 13 insertions(+), 5 deletions(-)
diff --git a/arch/powerpc/include/asm/thread_info.h
b/arch/powerpc/include/asm/thread_info.h
index 1e069a2e7ce87..31a7a157bc902 100644
--- a/arch/powerpc/include/asm/thread_info.h
+++ b/arch/powerpc/include/asm/thread_info.h
@@ -142,9 +142,6 @@ void arch_setup_new_exec(void);
#define _TIF_SYSCALL_TRACEPOINT (1<<TIF_SYSCALL_TRACEPOINT)
#define _TIF_EMULATE_STACK_STORE (1<<TIF_EMULATE_STACK_STORE)
#define _TIF_SYSCALL_EMU (1<<TIF_SYSCALL_EMU)
-#define _TIF_SYSCALL_DOTRACE (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
- _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT | \
- _TIF_SYSCALL_EMU)
#define _TIF_USER_WORK_MASK (_TIF_SIGPENDING | _TIF_NEED_RESCHED | \
_TIF_NEED_RESCHED_LAZY | _TIF_NOTIFY_RESUME | \
@@ -153,6 +150,18 @@ void arch_setup_new_exec(void);
#define _TIF_PERSYSCALL_MASK (_TIF_RESTOREALL|_TIF_NOERROR)
+/*
+ * Syscall work after which the exit must reload all registers from
+ * pt_regs: a tracer may have changed any of them at a syscall or seccomp
+ * stop, or under PTRACE_SYSEMU. Audit and the tracepoint are included to
+ * keep the pre-GENERIC_ENTRY behaviour.
+ */
+#define SYSCALL_WORK_RESTOREALL (SYSCALL_WORK_SECCOMP |
\
+ SYSCALL_WORK_SYSCALL_TRACEPOINT | \
+ SYSCALL_WORK_SYSCALL_TRACE | \
+ SYSCALL_WORK_SYSCALL_EMU | \
+ SYSCALL_WORK_SYSCALL_AUDIT)
+
/* Bits in local_flags */
/* Don't move TLF_NAPPING without adjusting the code in entry_32.S */
#define TLF_NAPPING 0 /* idle thread enabled NAP mode */
diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 55f9c0c9922ac..d6a7b6d2b4bd2 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -117,9 +117,8 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
regs->gpr[3] = r3;
}
- if (unlikely(ti_flags & _TIF_SYSCALL_DOTRACE)) {
+ if (unlikely(READ_ONCE(current_thread_info()->syscall_work) &
SYSCALL_WORK_RESTOREALL))
ret |= _TIF_RESTOREALL;
- }
syscall_exit_to_user_mode(regs);
--
2.51.0
