Re: [PATCH] x86-32: fix kexec with stack canary (CONFIG_CC_STACKPROTECTOR)

[Eric W. Biederman]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Date: Wed, 27 Dec 2017 11:41:30 -0800
> Subject: [PATCH] x86-32: fix kexec with stack canary (CONFIG_CC_STACKPROTECTOR)
>
> 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.
>
> Reported-and-tested-by: Alexandru Chirvasitu <achirvasub@xxxxxxxxx>
> Fixes: e802a51ede91 ("x86/idt: Consolidate IDT invalidation")
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Peter Anvin <hpa@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> ---
>
> I wrote "Reported-and-tested-by: Alexandru" because while this isn't 
> exactly the same patch as anything Alexandru tested, it's pretty close, 
> and I'm pretty sure this version will fix his issues too.
>
> I decided to try to just do the minimal changes: the GDT invalidation last 
> (because of the debugging) and _only_ removing the resetting of fs/gs 
> rather than removing load_segments() entirely.
>
> I think making idt_invalidate() be inline would be a good thing as well, 
> and I do think that all those "phys_to_virt(0)" things are garbage, but I 
> also think they are independent issues, so I didn't touch any of that. 
>
> I'm assuming I'll get this patch back through the x86 tree, and will not 
> be applying it to my own git tree unless the x86 people ask me to.
>
> Comments?

There is one significant problem with this patch.  It changes the ABI
that kexec provides to the next kernel.

That ABI is that the segments will be set to a well defined value.
That value is flat 32bit segments with a base address of 0.

By removing %fs and %gs from load_segments they are now effectively
random undefined values, to the next kernel.

I don't know if anything actually cares.  But if they do they are now
broken.  It is easy enough to preserve that invariant I don't see
a point in risking potential breaking and looking to see if we have
actually broken the ABI.

It feels like this is something we should move into assembly rather
than attempting to cater to the changing evironment of C code in the
kernel.  Or if not we need a big fat comment be very very careful
this code is special.

Eric


>  arch/x86/kernel/machine_kexec_32.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/machine_kexec_32.c b/arch/x86/kernel/machine_kexec_32.c
> index 00bc751c861c..edfede768688 100644
> --- 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,