From: Joerg Roedel <jroedel@xxxxxxx> Add a config option that enabled code to check that we enter and leave the kernel with the correct cr3. This is needed because we have no NX protection of user-addresses in the kernel-cr3 on x86-32 and wouldn't notice that type of bug otherwise. Signed-off-by: Joerg Roedel <jroedel@xxxxxxx> --- arch/x86/Kconfig.debug | 12 ++++++++++++ arch/x86/entry/entry_32.S | 43 +++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 55 insertions(+) diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug index 192e4d2..a57f556 100644 --- a/arch/x86/Kconfig.debug +++ b/arch/x86/Kconfig.debug @@ -337,6 +337,18 @@ config X86_DEBUG_FPU If unsure, say N. +config X86_DEBUG_ENTRY_CR3 + bool "Debug CR3 for Kernel entry/exit" + depends on X86_32 && PAGE_TABLE_ISOLATION + ---help--- + Add instructions to the x86-32 entry code to check whether the kernel + is entered and left with the correct CR3. When PTI is enabled, this + checks whether we enter the kernel with the user-space cr3 when + coming from user-mode and if we leave with user-cr3 back to + user-space. + + If unsure, say N. + config PUNIT_ATOM_DEBUG tristate "ATOM Punit debug driver" depends on PCI diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S index f47e535..6b371a9 100644 --- a/arch/x86/entry/entry_32.S +++ b/arch/x86/entry/entry_32.S @@ -166,6 +166,24 @@ .Lend_\@: .endm +.macro BUG_IF_WRONG_CR3 no_user_check=0 +#ifdef CONFIG_X86_DEBUG_ENTRY_CR3 + ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI + .if \no_user_check == 0 + /* coming from usermode? */ + testl $SEGMENT_RPL_MASK, PT_CS(%esp) + jz .Lend_\@ + .endif + /* On user-cr3? */ + movl %cr3, %eax + testl $PTI_SWITCH_MASK, %eax + jnz .Lend_\@ + /* From userspace with kernel cr3 - BUG */ + ud2 +.Lend_\@: +#endif +.endm + /* * Switch to kernel cr3 if not already loaded and return current cr3 in * \scratch_reg @@ -218,6 +236,8 @@ .macro SAVE_ALL_NMI cr3_reg:req SAVE_ALL + BUG_IF_WRONG_CR3 + /* * Now switch the CR3 when PTI is enabled. * @@ -229,6 +249,7 @@ .Lend_\@: .endm + /* * This is a sneaky trick to help the unwinder find pt_regs on the stack. The * frame pointer is replaced with an encoded pointer to pt_regs. The encoding @@ -292,6 +313,8 @@ .Lswitched_\@: + BUG_IF_WRONG_CR3 + RESTORE_REGS pop=\pop .endm @@ -362,6 +385,8 @@ ALTERNATIVE "", "jmp .Lend_\@", X86_FEATURE_XENPV + BUG_IF_WRONG_CR3 + SWITCH_TO_KERNEL_CR3 scratch_reg=%eax /* @@ -803,6 +828,7 @@ ENTRY(entry_SYSENTER_32) */ pushfl pushl %eax + BUG_IF_WRONG_CR3 no_user_check=1 SWITCH_TO_KERNEL_CR3 scratch_reg=%eax popl %eax popfl @@ -897,6 +923,7 @@ ENTRY(entry_SYSENTER_32) * whereas POPF does not.) */ btr $X86_EFLAGS_IF_BIT, (%esp) + BUG_IF_WRONG_CR3 no_user_check=1 popfl popl %eax @@ -974,6 +1001,8 @@ restore_all: /* Switch back to user CR3 */ SWITCH_TO_USER_CR3 scratch_reg=%eax + BUG_IF_WRONG_CR3 + /* Restore user state */ RESTORE_REGS pop=4 # skip orig_eax/error_code .Lirq_return: @@ -987,6 +1016,7 @@ restore_all: restore_all_kernel: TRACE_IRQS_IRET PARANOID_EXIT_TO_KERNEL_MODE + BUG_IF_WRONG_CR3 RESTORE_REGS 4 jmp .Lirq_return @@ -994,6 +1024,19 @@ restore_all_kernel: ENTRY(iret_exc ) pushl $0 # no error code pushl $do_iret_error + +#ifdef CONFIG_X86_DEBUG_ENTRY_CR3 + /* + * The stack-frame here is the one that iret faulted on, so its a + * return-to-user frame. We are on kernel-cr3 because we come here from + * the fixup code. This confuses the CR3 checker, so switch to user-cr3 + * as the checker expects it. + */ + pushl %eax + SWITCH_TO_USER_CR3 scratch_reg=%eax + popl %eax +#endif + jmp common_exception .previous _ASM_EXTABLE(.Lirq_return, iret_exc) -- 2.7.4
