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Subject[PATCH 4.20 151/183] parisc: Fix ptrace syscall number modification
Date
4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dmitry V. Levin <ldv@altlinux.org>

commit b7dc5a071ddf69c0350396b203cba32fe5bab510 upstream.

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>
Tested-by: Helge Deller <deller@gmx.de>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/parisc/kernel/ptrace.c | 29 +++++++++++++++++++++--------
1 file changed, 21 insertions(+), 8 deletions(-)

--- a/arch/parisc/kernel/ptrace.c
+++ b/arch/parisc/kernel/ptrace.c
@@ -308,15 +308,29 @@ long compat_arch_ptrace(struct task_stru

long do_syscall_trace_enter(struct pt_regs *regs)
{
- if (test_thread_flag(TIF_SYSCALL_TRACE) &&
- tracehook_report_syscall_entry(regs)) {
+ if (test_thread_flag(TIF_SYSCALL_TRACE)) {
+ int rc = 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.
+ * As tracesys_next does not set %r28 to -ENOSYS
+ * when %r20 is set to -1, initialize it here.
*/
- regs->gr[20] = -1UL;
- goto out;
+ regs->gr[28] = -ENOSYS;
+
+ if (rc) {
+ /*
+ * A nonzero return code from
+ * tracehook_report_syscall_entry() tells us
+ * to prevent the syscall execution. Skip
+ * the syscall call and the syscall restart handling.
+ *
+ * Note that the tracer may also just change
+ * regs->gr[20] to an invalid syscall number,
+ * that is handled by tracesys_next.
+ */
+ regs->gr[20] = -1UL;
+ return -1;
+ }
}

/* Do the secure computing check after ptrace. */
@@ -340,7 +354,6 @@ long do_syscall_trace_enter(struct pt_re
regs->gr[24] & 0xffffffff,
regs->gr[23] & 0xffffffff);

-out:
/*
* Sign extend the syscall number to 64bit since it may have been
* modified by a compat ptrace call

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