[PATCH 4.14 141/146] x86-32: Fix kexec with stack canary (CONFIG_CC_STACKPROTECTOR)

Greg Kroah-Hartman
2018-01-01T14:50:39Z

From: Linus Torvalds <torvalds@linux-foundation.org>

commit ac461122c88a10b7d775de2f56467f097c9e627a upstream.

Commit e802a51ede91 ("x86/idt: Consolidate IDT invalidation") cleaned up
and unified the IDT invalidation that existed in a couple of places. It
changed no actual real code.

Despite not changing any actual real code, it _did_ change code generation:
by implementing the common idt_invalidate() function in
archx86/kernel/idt.c, it made the use of the function in
arch/x86/kernel/machine_kexec_32.c be a real function call rather than an
(accidental) inlining of the function.

That, in turn, exposed two issues:

- in load_segments(), we had incorrectly reset all the segment
registers, which then made the stack canary load (which gcc does
using offset of %gs) cause a trap. Instead of %gs pointing to the
stack canary, it will be the normal zero-based kernel segment, and
the stack canary load will take a page fault at address 0x14.

- to make this even harder to debug, we had invalidated the GDT just
before calling idt_invalidate(), which meant that the fault happened
with an invalid GDT, which in turn causes a triple fault and
immediate reboot.

Fix this by

(a) not reloading the special segments in load_segments(). We currently
don't do any percpu accesses (which would require %fs on x86-32) in
this area, but there's no reason to think that we might not want to
do them, and like %gs, it's pointless to break it.

(b) doing idt_invalidate() before invalidating the GDT, to keep things
at least _slightly_ more debuggable for a bit longer. Without a
IDT, traps will not work. Without a GDT, traps also will not work,
but neither will any segment loads etc. So in a very real sense,
the GDT is even more core than the IDT.

Fixes: e802a51ede91 ("x86/idt: Consolidate IDT invalidation")
Reported-and-tested-by: Alexandru Chirvasitu <achirvasub@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/alpine.LFD.2.21.1712271143180.8572@i7.lan
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kernel/machine_kexec_32.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

--- a/arch/x86/kernel/machine_kexec_32.c
+++ b/arch/x86/kernel/machine_kexec_32.c
@@ -48,8 +48,6 @@ static void load_segments(void)
"\tmovl $"STR(__KERNEL_DS)",%%eax\n"
"\tmovl %%eax,%%ds\n"
"\tmovl %%eax,%%es\n"
- "\tmovl %%eax,%%fs\n"
- "\tmovl %%eax,%%gs\n"
"\tmovl %%eax,%%ss\n"
: : : "eax", "memory");
#undef STR
@@ -232,8 +230,8 @@ void machine_kexec(struct kimage *image)
* The gdt & idt are now invalid.
* If you want to load them you must set up your own idt & gdt.
*/
- set_gdt(phys_to_virt(0), 0);
idt_invalidate(phys_to_virt(0));
+ set_gdt(phys_to_virt(0), 0);

/* now call it */
image->start = relocate_kernel_ptr((unsigned long)image->head,