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SubjectRe: [PATCH 12/31] x86/entry/32: Add PTI cr3 switch to non-NMI entry/exit points
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On 02/09/2018 04:25 AM, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> Add unconditional cr3 switches between user and kernel cr3
> to all non-NMI entry and exit points.
>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> arch/x86/entry/entry_32.S | 59 ++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 58 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index 9693485..b5ef003 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -328,6 +328,25 @@
> #endif /* CONFIG_X86_ESPFIX32 */
> .endm
>
> +/* Unconditionally switch to user cr3 */
> +.macro SWITCH_TO_USER_CR3 scratch_reg:req
> + ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
> +
> + movl %cr3, \scratch_reg
> + orl $PTI_SWITCH_MASK, \scratch_reg
> + movl \scratch_reg, %cr3
> +.Lend_\@:
> +.endm
> +
> +/* Unconditionally switch to kernel cr3 */
> +.macro SWITCH_TO_KERNEL_CR3 scratch_reg:req
> + ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
> + movl %cr3, \scratch_reg
> + andl $(~PTI_SWITCH_MASK), \scratch_reg
> + movl \scratch_reg, %cr3
> +.Lend_\@:
> +.endm
> +
>
> /*
> * Called with pt_regs fully populated and kernel segments loaded,
> @@ -343,6 +362,8 @@
>
> ALTERNATIVE "", "jmp .Lend_\@", X86_FEATURE_XENPV
>
> + SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
> +
> /* Are we on the entry stack? Bail out if not! */
> movl PER_CPU_VAR(cpu_entry_area), %edi
> addl $CPU_ENTRY_AREA_entry_stack, %edi
> @@ -637,6 +658,18 @@ ENTRY(xen_sysenter_target)
> * 0(%ebp) arg6
> */
> ENTRY(entry_SYSENTER_32)
> + /*
> + * On entry-stack with all userspace-regs live - save and
> + * restore eflags and %eax to use it as scratch-reg for the cr3
> + * switch.
> + */
> + pushfl
> + pushl %eax
> + SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
> + popl %eax
> + popfl
> +
> + /* Stack empty again, switch to task stack */
> movl TSS_entry_stack(%esp), %esp
>
> .Lsysenter_past_esp:
> @@ -691,6 +724,10 @@ ENTRY(entry_SYSENTER_32)
> movl PT_OLDESP(%esp), %ecx /* pt_regs->sp */
> 1: mov PT_FS(%esp), %fs
> PTGS_TO_GS
> +
> + /* Segments are restored - switch to user cr3 */
> + SWITCH_TO_USER_CR3 scratch_reg=%eax
> +
> popl %ebx /* pt_regs->bx */
> addl $2*4, %esp /* skip pt_regs->cx and pt_regs->dx */
> popl %esi /* pt_regs->si */
> @@ -778,7 +815,23 @@ restore_all:
> .Lrestore_all_notrace:
> CHECK_AND_APPLY_ESPFIX
> .Lrestore_nocheck:
> - RESTORE_REGS 4 # skip orig_eax/error_code
> + /*
> + * First restore user segments. This can cause exceptions, so we
> + * run it with kernel cr3.
> + */
> + RESTORE_SEGMENTS
> +
> + /*
> + * Segments are restored - no more exceptions from here on except on
> + * iret, but that handled safely.
> + */
> + SWITCH_TO_USER_CR3 scratch_reg=%eax
> +
> + /* Restore rest */
> + RESTORE_INT_REGS
> +
> + /* Unwind stack to the iret frame */
> + RESTORE_SKIP_SEGMENTS 4 # skip orig_eax/error_code
> .Lirq_return:
> INTERRUPT_RETURN
>
> @@ -1139,6 +1192,10 @@ ENTRY(debug)
>
> SAVE_ALL
> ENCODE_FRAME_POINTER
> +
> + /* Make sure we are running on kernel cr3 */
> + SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
> +
> xorl %edx, %edx # error code 0
> movl %esp, %eax # pt_regs pointer
>

The debug exception calls ret_from_exception on exit. If coming from
userspace, the C function prepare_exit_to_usermode() will be called.
With the PTI-32 code, it means that function will be called with the
entry stack instead of the task stack. This can be problematic as macro
like current won't work anymore.

-Longman

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