Commit e802a51ede ("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: e802a51ede ("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>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/alpine.LFD.2.21.1712271143180.8572@i7.lan
ac461122c8