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>

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 84e3a4ef9719562f67d32a426aa60e5c23093abd..518bb99c339480820fc3995b1456d29704d67f07 100644 (file)
--- 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