On Wed, Jul 13, 2022 at 02:12:41PM -0300, Thadeu Lima de Souza Cascardo wrote:
The return thunk call makes the fastop functions larger, just like IBT does. Consider a 16-byte FASTOP_SIZE when CONFIG_RETHUNK is enabled.
Otherwise, functions will be incorrectly aligned and when computing their position for differently sized operators, they will executed in the middle or end of a function, which may as well be an int3, leading to a crash like:
Bah.. I did the SETcc stuff, but then forgot about the FASTOP :/
af2e140f3420 ("x86/kvm: Fix SETcc emulation for return thunks")
Fixes: aa3d480315ba ("x86: Use return-thunk in asm code") Signed-off-by: Thadeu Lima de Souza Cascardo cascardo@canonical.com Cc: Peter Zijlstra (Intel) peterz@infradead.org Cc: Borislav Petkov bp@suse.de Cc: Josh Poimboeuf jpoimboe@kernel.org Cc: Paolo Bonzini pbonzini@redhat.com Reported-by: Linux Kernel Functional Testing lkft@linaro.org
arch/x86/kvm/emulate.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index db96bf7d1122..d779eea1052e 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -190,7 +190,7 @@ #define X16(x...) X8(x), X8(x) #define NR_FASTOP (ilog2(sizeof(ulong)) + 1) -#define FASTOP_SIZE (8 * (1 + HAS_KERNEL_IBT)) +#define FASTOP_SIZE (8 * (1 + (HAS_KERNEL_IBT | IS_ENABLED(CONFIG_RETHUNK))))
Would it make sense to do something like this instead?
--- arch/x86/kvm/emulate.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index db96bf7d1122..b4305d2dcc51 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -189,8 +189,12 @@ #define X8(x...) X4(x), X4(x) #define X16(x...) X8(x), X8(x)
-#define NR_FASTOP (ilog2(sizeof(ulong)) + 1) -#define FASTOP_SIZE (8 * (1 + HAS_KERNEL_IBT)) +#define NR_FASTOP (ilog2(sizeof(ulong)) + 1) +#define RET_LENGTH (1 + (4 * IS_ENABLED(CONFIG_RETHUNK)) + \ + IS_ENABLED(CONFIG_SLS)) +#define FASTOP_LENGTH (7 + ENDBR_INSN_SIZE + RET_LENGTH) +#define FASTOP_SIZE (8 << ((FASTOP_LENGTH > 8) & 1) << ((FASTOP_LENGTH > 16) & 1)) +static_assert(FASTOP_LENGTH <= FASTOP_SIZE);
struct opcode { u64 flags; @@ -442,8 +446,6 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop); * RET | JMP __x86_return_thunk [1,5 bytes; CONFIG_RETHUNK] * INT3 [1 byte; CONFIG_SLS] */ -#define RET_LENGTH (1 + (4 * IS_ENABLED(CONFIG_RETHUNK)) + \ - IS_ENABLED(CONFIG_SLS)) #define SETCC_LENGTH (ENDBR_INSN_SIZE + 3 + RET_LENGTH) #define SETCC_ALIGN (4 << ((SETCC_LENGTH > 4) & 1) << ((SETCC_LENGTH > 8) & 1)) static_assert(SETCC_LENGTH <= SETCC_ALIGN);