Commit c63855d0 authored by Roland McGrath's avatar Roland McGrath Committed by Ingo Molnar
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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: default avatarRoland McGrath <>
Signed-off-by: default avatarThomas Gleixner <>
Signed-off-by: default avatarIngo Molnar <>
parent c1f766b5
......@@ -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;
*pt_regs_access(task_pt_regs(task), offset) = value;
else {
case offsetof(struct user_regs_struct, gs):
task-> = 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;
if (test_tsk_thread_flag(task, TIF_IA32))
task_pt_regs(task)->cs = value;
case offsetof(struct user_regs_struct,ss):
if (unlikely(value == 0))
return -EIO;
if (test_tsk_thread_flag(task, TIF_IA32))
task_pt_regs(task)->ss = value;
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