From 708078f65721b46d82d9934a3f0b36a2b8ad0656 Mon Sep 17 00:00:00 2001 From: "Chang S. Bae" Date: Wed, 8 May 2019 03:02:27 -0700 Subject: [PATCH] x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit Without FSGSBASE, user space cannot change GSBASE other than through a PRCTL. The kernel enforces that the user space GSBASE value is postive as negative values are used for detecting the kernel space GSBASE value in the paranoid entry code. If FSGSBASE is enabled, user space can set arbitrary GSBASE values without kernel intervention, including negative ones, which breaks the paranoid entry assumptions. To avoid this, paranoid entry needs to unconditionally save the current GSBASE value independent of the interrupted context, retrieve and write the kernel GSBASE and unconditionally restore the saved value on exit. The restore happens either in paranoid_exit or in the special exit path of the NMI low level code. All other entry code pathes which use unconditional SWAPGS are not affected as they do not depend on the actual content. [ tglx: Massaged changelogs and comments ] Suggested-by: H. Peter Anvin Suggested-by: Andy Lutomirski Suggested-by: Thomas Gleixner Signed-off-by: Chang S. Bae Signed-off-by: Thomas Gleixner Cc: Andi Kleen Cc: Ravi Shankar Cc: Dave Hansen Link: https://lkml.kernel.org/r/1557309753-24073-13-git-send-email-chang.seok.bae@intel.com --- arch/x86/entry/calling.h | 6 ++++ arch/x86/entry/entry_64.S | 80 ++++++++++++++++++++++++++++++++++++++++------- 2 files changed, 75 insertions(+), 11 deletions(-) diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h index 9a52436..d3fbe2d 100644 --- a/arch/x86/entry/calling.h +++ b/arch/x86/entry/calling.h @@ -338,6 +338,12 @@ For 32-bit we have the following conventions - kernel is built with #endif .endm +.macro SAVE_AND_SET_GSBASE scratch_reg:req save_reg:req + rdgsbase \save_reg + GET_PERCPU_BASE \scratch_reg + wrgsbase \scratch_reg +.endm + #endif /* CONFIG_X86_64 */ .macro STACKLEAK_ERASE diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index aaa846f..7f9f511 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -38,6 +38,7 @@ #include #include #include +#include #include #include "calling.h" @@ -947,7 +948,6 @@ ENTRY(\sym) addq $\ist_offset, CPU_TSS_IST(\shift_ist) .endif - /* these procedures expect "no swapgs" flag in ebx */ .if \paranoid jmp paranoid_exit .else @@ -1164,9 +1164,14 @@ idtentry machine_check do_mce has_error_code=0 paranoid=1 #endif /* - * Save all registers in pt_regs, and switch gs if needed. - * Use slow, but surefire "are we in kernel?" check. - * Return: ebx=0: need swapgs on exit, ebx=1: otherwise + * Save all registers in pt_regs. Return GSBASE related information + * in EBX depending on the availability of the FSGSBASE instructions: + * + * FSGSBASE R/EBX + * N 0 -> SWAPGS on exit + * 1 -> no SWAPGS on exit + * + * Y GSBASE value at entry, must be restored in paranoid_exit */ ENTRY(paranoid_entry) UNWIND_HINT_FUNC @@ -1174,7 +1179,6 @@ ENTRY(paranoid_entry) PUSH_AND_CLEAR_REGS save_ret=1 ENCODE_FRAME_POINTER 8 -1: /* * Always stash CR3 in %r14. This value will be restored, * verbatim, at exit. Needed if paranoid_entry interrupted @@ -1192,6 +1196,25 @@ ENTRY(paranoid_entry) */ SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14 + /* + * Handling GSBASE depends on the availability of FSGSBASE. + * + * Without FSGSBASE the kernel enforces that negative GSBASE + * values indicate kernel GSBASE. With FSGSBASE no assumptions + * can be made about the GSBASE value when entering from user + * space. + */ + ALTERNATIVE "jmp .Lparanoid_entry_checkgs", "", X86_FEATURE_FSGSBASE + + /* + * Read the current GSBASE and store it in in %rbx unconditionally, + * retrieve and set the current CPUs kernel GSBASE. The stored value + * has to be restored in paranoid_exit unconditionally. + */ + SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx + ret + +.Lparanoid_entry_checkgs: /* EBX = 1 -> kernel GSBASE active, no restore required */ movl $1, %ebx /* @@ -1218,16 +1241,32 @@ END(paranoid_entry) * * We may be returning to very strange contexts (e.g. very early * in syscall entry), so checking for preemption here would - * be complicated. Fortunately, we there's no good reason - * to try to handle preemption here. + * be complicated. Fortunately, there's no good reason to try + * to handle preemption here. * - * On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) + * R/EBX contains the GSBASE related information depending on the + * availability of the FSGSBASE instructions: + * + * FSGSBASE R/EBX + * N 0 -> SWAPGS on exit + * 1 -> no SWAPGS on exit + * + * Y User space GSBASE, must be restored unconditionally */ ENTRY(paranoid_exit) UNWIND_HINT_REGS DISABLE_INTERRUPTS(CLBR_ANY) TRACE_IRQS_OFF_DEBUG - /* If EBX is 0, SWAPGS is required */ + + /* Handle GS depending on FSGSBASE availability */ + ALTERNATIVE "jmp .Lparanoid_exit_checkgs", "nop",X86_FEATURE_FSGSBASE + + /* With FSGSBASE enabled, unconditionally restore GSBASE */ + wrgsbase %rbx + jmp .Lparanoid_exit_no_swapgs; + +.Lparanoid_exit_checkgs: + /* On non-FSGSBASE systems, conditionally do SWAPGS */ testl %ebx, %ebx jnz .Lparanoid_exit_no_swapgs TRACE_IRQS_IRETQ @@ -1235,12 +1274,14 @@ ENTRY(paranoid_exit) RESTORE_CR3 scratch_reg=%rbx save_reg=%r14 SWAPGS_UNSAFE_STACK jmp .Lparanoid_exit_restore + .Lparanoid_exit_no_swapgs: TRACE_IRQS_IRETQ_DEBUG /* Always restore stashed CR3 value (see paranoid_entry) */ RESTORE_CR3 scratch_reg=%rbx save_reg=%r14 + .Lparanoid_exit_restore: - jmp restore_regs_and_return_to_kernel + jmp restore_regs_and_return_to_kernel END(paranoid_exit) /* @@ -1651,10 +1692,27 @@ end_repeat_nmi: /* Always restore stashed CR3 value (see paranoid_entry) */ RESTORE_CR3 scratch_reg=%r15 save_reg=%r14 - testl %ebx, %ebx /* swapgs needed? */ + /* + * The above invocation of paranoid_entry stored the GSBASE + * related information in R/EBX depending on the availability + * of FSGSBASE. + * + * If FSGSBASE is enabled, restore the saved GSBASE value + * unconditionally, otherwise take the conditional SWAPGS path. + */ + ALTERNATIVE "jmp nmi_no_fsgsbase", "", X86_FEATURE_FSGSBASE + + wrgsbase %rbx + jmp nmi_restore + +nmi_no_fsgsbase: + /* EBX == 0 -> invoke SWAPGS */ + testl %ebx, %ebx jnz nmi_restore + nmi_swapgs: SWAPGS_UNSAFE_STACK + nmi_restore: POP_REGS -- 2.7.4