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Subject[PATCH] parisc: Fix ptrace syscall number modification
Commit 910cd32e552e ("parisc: Fix and enable seccomp filter support")
introduced a regression in ptrace-based syscall tampering: when tracer
changes syscall number to -1, the kernel fails to initialize %r28 with
-ENOSYS and subsequently fails to return the error code of the failed
syscall to userspace.

This erroneous behaviour could be observed with a simple strace syscall
fault injection command which is expected to print something like this:

$ strace -a0 -ewrite -einject=write:error=enospc echo hello
write(1, "hello\n", 6) = -1 ENOSPC (No space left on device) (INJECTED)
write(2, "echo: ", 6) = -1 ENOSPC (No space left on device) (INJECTED)
write(2, "write error", 11) = -1 ENOSPC (No space left on device) (INJECTED)
write(2, "\n", 1) = -1 ENOSPC (No space left on device) (INJECTED)
+++ exited with 1 +++

After commit 910cd32e552ea09caa89cdbe328e468979b030dd it loops printing
something like this instead:

write(1, "hello\n", 6../strace: Failed to tamper with process 12345: unexpectedly got no error (return value 0, error 0)
) = 0 (INJECTED)

This bug was found by strace test suite.

Fixes: 910cd32e552e ("parisc: Fix and enable seccomp filter support")
Cc: stable@vger.kernel.org # v4.5+
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
---

N.B. I have no parisc box to test the patch.

arch/parisc/kernel/ptrace.c | 20 +++++++++++---------
1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/arch/parisc/kernel/ptrace.c b/arch/parisc/kernel/ptrace.c
index 2582df1c529b..9177f3e68b93 100644
--- a/arch/parisc/kernel/ptrace.c
+++ b/arch/parisc/kernel/ptrace.c
@@ -308,15 +308,17 @@ long compat_arch_ptrace(struct task_struct *child, compat_long_t request,

long do_syscall_trace_enter(struct pt_regs *regs)
{
- if (test_thread_flag(TIF_SYSCALL_TRACE) &&
- tracehook_report_syscall_entry(regs)) {
- /*
- * Tracing decided this syscall should not happen or the
- * debugger stored an invalid system call number. Skip
- * the system call and the system call restart handling.
- */
- regs->gr[20] = -1UL;
- goto out;
+ if (test_thread_flag(TIF_SYSCALL_TRACE)) {
+ regs->gr[28] = -ENOSYS;
+ if (tracehook_report_syscall_entry(regs)) {
+ /*
+ * Tracing decided this syscall should not happen or the
+ * debugger stored an invalid system call number. Skip
+ * the system call and the system call restart handling.
+ */
+ regs->gr[20] = -1UL;
+ return -1;
+ }
}

/* Do the secure computing check after ptrace. */
--
ldv
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