[34-longterm 081/260] x86-64, compat: Retruncate rax after ia32 syscall entry tracing

[Paul Gortmaker]

From: Roland McGrath <roland@xxxxxxxxxx>

commit eefdca043e8391dcd719711716492063030b55ac upstream.

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@xxxxxxxxxxx>
Signed-off-by: Roland McGrath <roland@xxxxxxxxxx>
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
 arch/x86/ia32/ia32entry.S |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 944e3fc..4f5f71e 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
-- 
1.7.3.3

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