The patch below does not apply to the 4.19-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From a1a338e5b6fe9e0a39c57c232dc96c198bb53e47 Mon Sep 17 00:00:00 2001
From: Andy Lutomirski luto@kernel.org Date: Wed, 20 Nov 2019 10:10:49 +0100 Subject: [PATCH] x86/entry/32: Unwind the ESPFIX stack earlier on exception entry
Right now, we do some fancy parts of the exception entry path while SS might have a nonzero base: we fill in regs->ss and regs->sp, and we consider switching to the kernel stack. This results in regs->ss and regs->sp referring to a non-flat stack and it may result in overflowing the entry stack. The former issue means that we can try to call iret_exc on a non-flat stack, which doesn't work.
Tested with selftests/x86/sigreturn_32.
Fixes: 45d7b255747c ("x86/entry/32: Enter the kernel via trampoline stack") Signed-off-by: Andy Lutomirski luto@kernel.org Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: stable@kernel.org
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S index d9f401995278..647e2a272d08 100644 --- a/arch/x86/entry/entry_32.S +++ b/arch/x86/entry/entry_32.S @@ -210,8 +210,6 @@ /* * The high bits of the CS dword (__csh) are used for CS_FROM_*. * Clear them in case hardware didn't do this for us. - * - * Be careful: we may have nonzero SS base due to ESPFIX. */ andl $0x0000ffff, 4*4(%esp)
@@ -307,12 +305,21 @@ .Lfinished_frame_@: .endm
-.macro SAVE_ALL pt_regs_ax=%eax switch_stacks=0 skip_gs=0 +.macro SAVE_ALL pt_regs_ax=%eax switch_stacks=0 skip_gs=0 unwind_espfix=0 cld .if \skip_gs == 0 PUSH_GS .endif pushl %fs + + pushl %eax + movl $(__KERNEL_PERCPU), %eax + movl %eax, %fs +.if \unwind_espfix > 0 + UNWIND_ESPFIX_STACK +.endif + popl %eax + FIXUP_FRAME pushl %es pushl %ds @@ -326,8 +333,6 @@ movl $(__USER_DS), %edx movl %edx, %ds movl %edx, %es - movl $(__KERNEL_PERCPU), %edx - movl %edx, %fs .if \skip_gs == 0 SET_KERNEL_GS %edx .endif @@ -1153,18 +1158,17 @@ ENDPROC(entry_INT80_32) lss (%esp), %esp /* switch to the normal stack segment */ #endif .endm + .macro UNWIND_ESPFIX_STACK + /* It's safe to clobber %eax, all other regs need to be preserved */ #ifdef CONFIG_X86_ESPFIX32 movl %ss, %eax /* see if on espfix stack */ cmpw $__ESPFIX_SS, %ax - jne 27f - movl $__KERNEL_DS, %eax - movl %eax, %ds - movl %eax, %es + jne .Lno_fixup_@ /* switch to normal stack */ FIXUP_ESPFIX_STACK -27: +.Lno_fixup_@: #endif .endm
@@ -1458,10 +1462,9 @@ END(page_fault)
common_exception_read_cr2: /* the function address is in %gs's slot on the stack */ - SAVE_ALL switch_stacks=1 skip_gs=1 + SAVE_ALL switch_stacks=1 skip_gs=1 unwind_espfix=1
ENCODE_FRAME_POINTER - UNWIND_ESPFIX_STACK
/* fixup %gs */ GS_TO_REG %ecx @@ -1483,9 +1486,8 @@ END(common_exception_read_cr2)
common_exception: /* the function address is in %gs's slot on the stack */ - SAVE_ALL switch_stacks=1 skip_gs=1 + SAVE_ALL switch_stacks=1 skip_gs=1 unwind_espfix=1 ENCODE_FRAME_POINTER - UNWIND_ESPFIX_STACK
/* fixup %gs */ GS_TO_REG %ecx