Commit 5c8f6a2e authored by Lai Jiangshan's avatar Lai Jiangshan Committed by Borislav Petkov
Browse files

x86/xen: Add xenpv_restore_regs_and_return_to_usermode() 



In the native case, PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is the
trampoline stack. But XEN pv doesn't use trampoline stack, so
PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is also the kernel stack.

In that case, source and destination stacks are identical, which means
that reusing swapgs_restore_regs_and_return_to_usermode() in XEN pv
would cause %rsp to move up to the top of the kernel stack and leave the
IRET frame below %rsp.

This is dangerous as it can be corrupted if #NMI / #MC hit as either of
these events occurring in the middle of the stack pushing would clobber
data on the (original) stack.

And, with  XEN pv, swapgs_restore_regs_and_return_to_usermode() pushing
the IRET frame on to the original address is useless and error-prone
when there is any future attempt to modify the code.

 [ bp: Massage commit message. ]

Fixes: 7f2590a1 ("x86/entry/64: Use a per-CPU trampoline stack for IDT entries")
Signed-off-by: default avatarLai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Reviewed-by: default avatarBoris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lkml.kernel.org/r/20211126101209.8613-4-jiangshanlai@gmail.com
parent 1367afaa

arch/x86/entry/entry_64.S

+4 −0
Original line number Diff line number Diff line
@@ -574,6 +574,10 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL) 
	ud2

1:

#endif

#ifdef CONFIG_XEN_PV

	ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV

#endif



	POP_REGS pop_rdi=0



	/*

arch/x86/xen/xen-asm.S

+20 −0
Original line number Diff line number Diff line
@@ -20,6 +20,7 @@ 


#include <linux/init.h>

#include <linux/linkage.h>

#include <../entry/calling.h>



.pushsection .noinstr.text, "ax"

/*
@@ -192,6 +193,25 @@ SYM_CODE_START(xen_iret) 
	jmp hypercall_iret

SYM_CODE_END(xen_iret)



/*

 * XEN pv doesn't use trampoline stack, PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is

 * also the kernel stack.  Reusing swapgs_restore_regs_and_return_to_usermode()

 * in XEN pv would cause %rsp to move up to the top of the kernel stack and

 * leave the IRET frame below %rsp, which is dangerous to be corrupted if #NMI

 * interrupts. And swapgs_restore_regs_and_return_to_usermode() pushing the IRET

 * frame at the same address is useless.

 */

SYM_CODE_START(xenpv_restore_regs_and_return_to_usermode)

	UNWIND_HINT_REGS

	POP_REGS



	/* stackleak_erase() can work safely on the kernel stack. */

	STACKLEAK_ERASE_NOCLOBBER



	addq	$8, %rsp	/* skip regs->orig_ax */

	jmp xen_iret

SYM_CODE_END(xenpv_restore_regs_and_return_to_usermode)



/*

 * Xen handles syscall callbacks much like ordinary exceptions, which

 * means we have: