3.16.40-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> commit 72b4f6a5e903b071f2a7c4eb1418cbe4eefdc344 upstream. On x86_32, when an interrupt happens from kernel space, SS and SP aren't pushed and the existing stack is used. So pt_regs is effectively two words shorter, and the previous stack pointer is normally the memory after the shortened pt_regs, aka '®s->sp'. But in the rare case where the interrupt hits right after the stack pointer has been changed to point to an empty stack, like for example when call_on_stack() is used, the address immediately after the shortened pt_regs is no longer on the stack. In that case, instead of '®s->sp', the previous stack pointer should be retrieved from the beginning of the current stack page. kernel_stack_pointer() wants to do that, but it forgets to dereference the pointer. So instead of returning a pointer to the previous stack, it returns a pointer to the beginning of the current stack. Note that it's probably outside of kernel_stack_pointer()'s scope to be switching stacks at all. The x86_64 version of this function doesn't do it, and it would be better for the caller to do it if necessary. But that's a patch for another day. This just fixes the original intent. Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Brian Gerst <brgerst@xxxxxxxxx> Cc: Byungchul Park <byungchul.park@xxxxxxx> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Nilay Vaish <nilayvaish@xxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Fixes: 0788aa6a23cb ("x86: Prepare removal of previous_esp from i386 thread_info structure") Link: http://lkml.kernel.org/r/472453d6e9f6a2d4ab16aaed4935f43117111566.1471535549.git.jpoimboe@xxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> --- arch/x86/kernel/ptrace.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/arch/x86/kernel/ptrace.c +++ b/arch/x86/kernel/ptrace.c @@ -190,8 +190,8 @@ unsigned long kernel_stack_pointer(struc return sp; prev_esp = (u32 *)(context); - if (prev_esp) - return (unsigned long)prev_esp; + if (*prev_esp) + return (unsigned long)*prev_esp; return (unsigned long)regs; }