4.14-stable review patch. If anyone has any objections, please let me know.
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From: Ingo Molnar mingo@kernel.org
commit e48657573481a5dff7cfdc3d57005c80aa816500 upstream.
Josh Poimboeuf noticed the following bug:
"The paranoid exit code only restores the saved CR3 when it switches back to the user GS. However, even in the kernel GS case, it's possible that it needs to restore a user CR3, if for example, the paranoid exception occurred in the syscall exit path between SWITCH_TO_USER_CR3_STACK and SWAPGS."
Josh also confirmed via targeted testing that it's possible to hit this bug.
Fix the bug by also restoring CR3 in the paranoid_exit_no_swapgs branch.
The reason we haven't seen this bug reported by users yet is probably because "paranoid" entry points are limited to the following cases:
idtentry double_fault do_double_fault has_error_code=1 paranoid=2 idtentry debug do_debug has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK idtentry int3 do_int3 has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK idtentry machine_check do_mce has_error_code=0 paranoid=1
Amongst those entry points only machine_check is one that will interrupt an IRQS-off critical section asynchronously - and machine check events are rare.
The other main asynchronous entries are NMI entries, which can be very high-freq with perf profiling, but they are special: they don't use the 'idtentry' macro but are open coded and restore user CR3 unconditionally so don't have this bug.
Reported-and-tested-by: Josh Poimboeuf jpoimboe@redhat.com Reviewed-by: Andy Lutomirski luto@kernel.org Acked-by: Thomas Gleixner tglx@linutronix.de Cc: Arjan van de Ven arjan@linux.intel.com Cc: Borislav Petkov bp@alien8.de Cc: Dan Williams dan.j.williams@intel.com Cc: Dave Hansen dave.hansen@linux.intel.com Cc: David Woodhouse dwmw2@infradead.org Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Link: http://lkml.kernel.org/r/20180214073910.boevmg65upbk3vqb@gmail.com Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/entry/entry_64.S | 1 + 1 file changed, 1 insertion(+)
--- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -1162,6 +1162,7 @@ ENTRY(paranoid_exit) jmp .Lparanoid_exit_restore .Lparanoid_exit_no_swapgs: TRACE_IRQS_IRETQ_DEBUG + RESTORE_CR3 scratch_reg=%rbx save_reg=%r14 .Lparanoid_exit_restore: jmp restore_regs_and_return_to_kernel END(paranoid_exit)