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Subject[PATCH] sh: fix syscall tracing
Addition of SECCOMP_FILTER exposed a longstanding bug in
do_syscall_trace_enter, whereby r0 (the 5th argument register) was
mistakenly used where r3 (syscall_nr) was intended. By overwriting r0
rather than r3 with -1 when attempting to block a syscall, the
existing code would instead have caused the syscall to execute with an
argument clobbered.

Commit 0bb605c2c7f2b4b3 then introduced skipping of the syscall when
do_syscall_trace_enter returns -1, so that the return value set by
seccomp filters would not be clobbered by -ENOSYS. This eliminated the
clobbering of the 5th argument register, but instead caused syscalls
made with a 5th argument of -1 to be misinterpreted as a request by
do_syscall_trace_enter to suppress the syscall.

Fixes: 0bb605c2c7f2b4b3 ("sh: Add SECCOMP_FILTER")
Fixes: ab99c733ae73cce3 ("sh: Make syscall tracer use tracehook notifiers, add TIF_NOTIFY_RESUME.")
Signed-off-by: Rich Felker <dalias@libc.org>
---
arch/sh/kernel/entry-common.S | 1 -
arch/sh/kernel/ptrace_32.c | 15 +++++----------
2 files changed, 5 insertions(+), 11 deletions(-)

diff --git a/arch/sh/kernel/entry-common.S b/arch/sh/kernel/entry-common.S
index ad963104d22d..91ab2607a1ff 100644
--- a/arch/sh/kernel/entry-common.S
+++ b/arch/sh/kernel/entry-common.S
@@ -370,7 +370,6 @@ syscall_trace_entry:
nop
cmp/eq #-1, r0
bt syscall_exit
- mov.l r0, @(OFF_R0,r15) ! Save return value
! Reload R0-R4 from kernel stack, where the
! parent may have modified them using
! ptrace(POKEUSR). (Note that R0-R2 are
diff --git a/arch/sh/kernel/ptrace_32.c b/arch/sh/kernel/ptrace_32.c
index b05bf92f9c32..5281685f6ad1 100644
--- a/arch/sh/kernel/ptrace_32.c
+++ b/arch/sh/kernel/ptrace_32.c
@@ -455,16 +455,11 @@ long arch_ptrace(struct task_struct *child, long request,

asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
{
- long ret = 0;
-
if (test_thread_flag(TIF_SYSCALL_TRACE) &&
- tracehook_report_syscall_entry(regs))
- /*
- * Tracing decided this syscall should not happen.
- * We'll return a bogus call number to get an ENOSYS
- * error, but leave the original number in regs->regs[0].
- */
- ret = -1L;
+ tracehook_report_syscall_entry(regs)) {
+ regs->regs[0] = -ENOSYS;
+ return -1;
+ }

if (secure_computing() == -1)
return -1;
@@ -475,7 +470,7 @@ asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
audit_syscall_entry(regs->regs[3], regs->regs[4], regs->regs[5],
regs->regs[6], regs->regs[7]);

- return ret ?: regs->regs[0];
+ return 0;
}

asmlinkage void do_syscall_trace_leave(struct pt_regs *regs)
--
2.21.0
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