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Subject[PATCH] x86 ptrace: disallow null cs/ss
Date

In my revamp of the x86 ptrace code for setting register values,
I accidentally omitted a check that was there in the old code.
Allowing %cs to be 0 causes a bad crash in recovery from iret failure.
This patch fixes that regression against 2.6.24, and adds a comment
that should help prevent this subtlety from being overlooked again.

Signed-off-by: Roland McGrath <roland@redhat.com>
---
arch/x86/kernel/ptrace.c | 25 +++++++++++++++++++++++--
1 files changed, 23 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 96286df..702c33e 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -103,9 +103,26 @@ static int set_segment_reg(struct task_struct *task,
if (invalid_selector(value))
return -EIO;

- if (offset != offsetof(struct user_regs_struct, gs))
+ /*
+ * For %cs and %ss we cannot permit a null selector.
+ * We can permit a bogus selector as long as it has USER_RPL.
+ * Null selectors are fine for other segment registers, but
+ * we will never get back to user mode with invalid %cs or %ss
+ * and will take the trap in iret instead. Much code relies
+ * on user_mode() to distinguish a user trap frame (which can
+ * safely use invalid selectors) from a kernel trap frame.
+ */
+ switch (offset) {
+ case offsetof(struct user_regs_struct, cs):
+ case offsetof(struct user_regs_struct, ss):
+ if (unlikely(value == 0))
+ return -EIO;
+
+ default:
*pt_regs_access(task_pt_regs(task), offset) = value;
- else {
+ break;
+
+ case offsetof(struct user_regs_struct, gs):
task->thread.gs = value;
if (task == current)
/*
@@ -227,12 +244,16 @@ static int set_segment_reg(struct task_struct *task,
* Can't actually change these in 64-bit mode.
*/
case offsetof(struct user_regs_struct,cs):
+ if (unlikely(value == 0))
+ return -EIO;
#ifdef CONFIG_IA32_EMULATION
if (test_tsk_thread_flag(task, TIF_IA32))
task_pt_regs(task)->cs = value;
#endif
break;
case offsetof(struct user_regs_struct,ss):
+ if (unlikely(value == 0))
+ return -EIO;
#ifdef CONFIG_IA32_EMULATION
if (test_tsk_thread_flag(task, TIF_IA32))
task_pt_regs(task)->ss = value;

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