diff options
Diffstat (limited to 'arch/x86/entry')
| -rw-r--r-- | arch/x86/entry/calling.h | 6 | ||||
| -rw-r--r-- | arch/x86/entry/entry_64.S | 111 |
2 files changed, 91 insertions, 26 deletions
diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h index 5c0cbb443030..98e4d8886f11 100644 --- a/arch/x86/entry/calling.h +++ b/arch/x86/entry/calling.h @@ -342,6 +342,12 @@ 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 + #else /* CONFIG_X86_64 */ # undef UNWIND_HINT_IRET_REGS # define UNWIND_HINT_IRET_REGS diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 04d1eea3ac3b..fb729f4c4fbc 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -38,6 +38,7 @@ #include <asm/frame.h> #include <asm/trapnr.h> #include <asm/nospec-branch.h> +#include <asm/fsgsbase.h> #include <linux/err.h> #include "calling.h" @@ -426,10 +427,7 @@ SYM_CODE_START(\asmsym) testb $3, CS-ORIG_RAX(%rsp) jnz .Lfrom_usermode_switch_stack_\@ - /* - * paranoid_entry returns SWAPGS flag for paranoid_exit in EBX. - * EBX == 0 -> SWAPGS, EBX == 1 -> no SWAPGS - */ + /* paranoid_entry returns GS information for paranoid_exit in EBX. */ call paranoid_entry UNWIND_HINT_REGS @@ -458,10 +456,7 @@ SYM_CODE_START(\asmsym) UNWIND_HINT_IRET_REGS offset=8 ASM_CLAC - /* - * paranoid_entry returns SWAPGS flag for paranoid_exit in EBX. - * EBX == 0 -> SWAPGS, EBX == 1 -> no SWAPGS - */ + /* paranoid_entry returns GS information for paranoid_exit in EBX. */ call paranoid_entry UNWIND_HINT_REGS @@ -798,9 +793,14 @@ SYM_CODE_END(xen_failsafe_callback) #endif /* CONFIG_XEN_PV */ /* - * 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 */ SYM_CODE_START_LOCAL(paranoid_entry) UNWIND_HINT_FUNC @@ -808,7 +808,6 @@ SYM_CODE_START_LOCAL(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 @@ -826,6 +825,28 @@ SYM_CODE_START_LOCAL(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 %rbx unconditionally, + * retrieve and set the current CPUs kernel GSBASE. The stored value + * has to be restored in paranoid_exit unconditionally. + * + * The MSR write ensures that no subsequent load is based on a + * mispredicted GSBASE. No extra FENCE required. + */ + 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 /* @@ -860,24 +881,45 @@ SYM_CODE_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. + * + * 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 * - * On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) + * Y User space GSBASE, must be restored unconditionally */ SYM_CODE_START_LOCAL(paranoid_exit) UNWIND_HINT_REGS - /* If EBX is 0, SWAPGS is required */ - testl %ebx, %ebx - jnz .Lparanoid_exit_no_swapgs - /* Always restore stashed CR3 value (see paranoid_entry) */ - RESTORE_CR3 scratch_reg=%rbx save_reg=%r14 + /* + * The order of operations is important. RESTORE_CR3 requires + * kernel GSBASE. + * + * NB to anyone to try to optimize this code: this code does + * not execute at all for exceptions from user mode. Those + * exceptions go through error_exit instead. + */ + RESTORE_CR3 scratch_reg=%rax save_reg=%r14 + + /* Handle the three GSBASE cases */ + ALTERNATIVE "jmp .Lparanoid_exit_checkgs", "", X86_FEATURE_FSGSBASE + + /* With FSGSBASE enabled, unconditionally restore GSBASE */ + wrgsbase %rbx + jmp restore_regs_and_return_to_kernel + +.Lparanoid_exit_checkgs: + /* On non-FSGSBASE systems, conditionally do SWAPGS */ + testl %ebx, %ebx + jnz restore_regs_and_return_to_kernel + + /* We are returning to a context with user GSBASE */ SWAPGS_UNSAFE_STACK - jmp restore_regs_and_return_to_kernel -.Lparanoid_exit_no_swapgs: - /* Always restore stashed CR3 value (see paranoid_entry) */ - RESTORE_CR3 scratch_reg=%rbx save_reg=%r14 - jmp restore_regs_and_return_to_kernel + jmp restore_regs_and_return_to_kernel SYM_CODE_END(paranoid_exit) /* @@ -1282,10 +1324,27 @@ 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 |