If both the tracer and the tracee are compat processes, and gprs[2]
is assigned a value by __poke_user_compat, then the higher 32 bits
of gprs[2] are cleared, IS_ERR_VALUE() always returns false, and
syscall_get_error() always returns 0.
Fix the implementation by sign-extending the value for compat processes
the same way as x86 implementation does.
The bug was exposed to user space by commit 201766a20e30f ("ptrace: add
PTRACE_GET_SYSCALL_INFO request") and detected by strace test suite.
This change fixes strace syscall tampering on s390.
Fixes: 753c4dd6a2fa2 ("[S390] ptrace changes")
Cc: Elvira Khabirova <lineprinter@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v2.6.28+
Signed-off-by: Dmitry V. Levin <ldv@xxxxxxxxxxxx>
---
arch/s390/include/asm/syscall.h | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/arch/s390/include/asm/syscall.h b/arch/s390/include/asm/syscall.h
index f073292e9fdb..c07633824715 100644
--- a/arch/s390/include/asm/syscall.h
+++ b/arch/s390/include/asm/syscall.h
@@ -33,7 +33,17 @@ static inline void syscall_rollback(struct task_struct *task,
static inline long syscall_get_error(struct task_struct *task,
struct pt_regs *regs)
{
- return IS_ERR_VALUE(regs->gprs[2]) ? regs->gprs[2] : 0;
+ unsigned long error = regs->gprs[2];
+#ifdef CONFIG_COMPAT
+ if (test_tsk_thread_flag(task, TIF_31BIT)) {
+ /*
+ * Sign-extend the value so (int)-EFOO becomes (long)-EFOO
+ * and will match correctly in comparisons.
+ */
+ error = (long) (int) error;
+ }
+#endif
+ return IS_ERR_VALUE(error) ? error : 0;
}
static inline long syscall_get_return_value(struct task_struct *task,
--
ldv
