x86-64, compat: Retruncate rax after ia32 syscall entry tracing

In commit d4d6715, we reopened an old hole for a 64-bit ptracer touching a
32-bit tracee in system call entry.  A %rax value set via ptrace at the
entry tracing stop gets used whole as a 32-bit syscall number, while we
only check the low 32 bits for validity.

Fix it by truncating %rax back to 32 bits after syscall_trace_enter,
in addition to testing the full 64 bits as has already been added.

Reported-by: Ben Hawkes <hawkes@sota.gen.nz>
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 84e3a4ef9719..518bb99c3394 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -50,7 +50,12 @@
         /*
          * Reload arg registers from stack in case ptrace changed them.
          * We don't reload %eax because syscall_trace_enter() returned
-         * the value it wants us to use in the table lookup.
+         * the %rax value we should see.  Instead, we just truncate that
+         * value to 32 bits again as we did on entry from user mode.
+         * If it's a new value set by user_regset during entry tracing,
+         * this matches the normal truncation of the user-mode value.
+         * If it's -1 to make us punt the syscall, then (u32)-1 is still
+         * an appropriately invalid value.
          */
         .macro LOAD_ARGS32 offset, _r9=0
         .if \_r9
@@ -60,6 +65,7 @@
         movl \offset+48(%rsp),%edx
         movl \offset+56(%rsp),%esi
         movl \offset+64(%rsp),%edi
+        movl %eax,%eax                  /* zero extension */
         .endm
         
         .macro CFI_STARTPROC32 simple