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authorRoland McGrath <roland@redhat.com>2008-02-06 22:39:44 +0100
committerIngo Molnar <mingo@elte.hu>2008-02-06 22:39:44 +0100
commitc63855d04034c96db791a7217954c93aa66d24cb (patch)
tree97f22e5bf68de576cad776cd14921903f7d51449
parentc1f766b5519f9b5a51b0e6884ed9e02bce775ea8 (diff)
x86 ptrace: disallow null cs/ss
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> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Ingo Molnar <mingo@elte.hu>
-rw-r--r--arch/x86/kernel/ptrace.c25
1 files changed, 23 insertions, 2 deletions
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 96286df1bb8..702c33efea8 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;