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    Subject[PATCH 5.15 32/78] x86,static_call: Use alternative RET encoding
    Date
    From: Peter Zijlstra <peterz@infradead.org>

    commit ee88d363d15617ff50ac24fab0ffec11113b2aeb upstream.

    In addition to teaching static_call about the new way to spell 'RET',
    there is an added complication in that static_call() is allowed to
    rewrite text before it is known which particular spelling is required.

    In order to deal with this; have a static_call specific fixup in the
    apply_return() 'alternative' patching routine that will rewrite the
    static_call trampoline to match the definite sequence.

    This in turn creates the problem of uniquely identifying static call
    trampolines. Currently trampolines are 8 bytes, the first 5 being the
    jmp.d32/ret sequence and the final 3 a byte sequence that spells out
    'SCT'.

    This sequence is used in __static_call_validate() to ensure it is
    patching a trampoline and not a random other jmp.d32. That is,
    false-positives shouldn't be plenty, but aren't a big concern.

    OTOH the new __static_call_fixup() must not have false-positives, and
    'SCT' decodes to the somewhat weird but semi plausible sequence:

    push %rbx
    rex.XB push %r12

    Additionally, there are SLS concerns with immediate jumps. Combined it
    seems like a good moment to change the signature to a single 3 byte
    trap instruction that is unique to this usage and will not ever get
    generated by accident.

    As such, change the signature to: '0x0f, 0xb9, 0xcc', which decodes
    to:

    ud1 %esp, %ecx

    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    [cascardo: skip validation as introduced by 2105a92748e8 ("static_call,x86: Robustify trampoline patching")]
    Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/x86/include/asm/static_call.h | 17 ++++++++++++++++
    arch/x86/kernel/alternative.c | 12 +++++++----
    arch/x86/kernel/static_call.c | 38 ++++++++++++++++++++++++++++++++++++-
    3 files changed, 62 insertions(+), 5 deletions(-)

    --- a/arch/x86/include/asm/static_call.h
    +++ b/arch/x86/include/asm/static_call.h
    @@ -21,6 +21,16 @@
    * relative displacement across sections.
    */

    +/*
    + * The trampoline is 8 bytes and of the general form:
    + *
    + * jmp.d32 \func
    + * ud1 %esp, %ecx
    + *
    + * That trailing #UD provides both a speculation stop and serves as a unique
    + * 3 byte signature identifying static call trampolines. Also see tramp_ud[]
    + * and __static_call_fixup().
    + */
    #define __ARCH_DEFINE_STATIC_CALL_TRAMP(name, insns) \
    asm(".pushsection .static_call.text, \"ax\" \n" \
    ".align 4 \n" \
    @@ -34,8 +44,13 @@
    #define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func) \
    __ARCH_DEFINE_STATIC_CALL_TRAMP(name, ".byte 0xe9; .long " #func " - (. + 4)")

    +#ifdef CONFIG_RETPOLINE
    +#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) \
    + __ARCH_DEFINE_STATIC_CALL_TRAMP(name, "jmp __x86_return_thunk")
    +#else
    #define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) \
    __ARCH_DEFINE_STATIC_CALL_TRAMP(name, "ret; int3; nop; nop; nop")
    +#endif


    #define ARCH_ADD_TRAMP_KEY(name) \
    @@ -44,4 +59,6 @@
    ".long " STATIC_CALL_KEY_STR(name) " - . \n" \
    ".popsection \n")

    +extern bool __static_call_fixup(void *tramp, u8 op, void *dest);
    +
    #endif /* _ASM_STATIC_CALL_H */
    --- a/arch/x86/kernel/alternative.c
    +++ b/arch/x86/kernel/alternative.c
    @@ -538,18 +538,22 @@ void __init_or_module noinline apply_ret
    s32 *s;

    for (s = start; s < end; s++) {
    - void *addr = (void *)s + *s;
    + void *dest = NULL, *addr = (void *)s + *s;
    struct insn insn;
    int len, ret;
    u8 bytes[16];
    - u8 op1;
    + u8 op;

    ret = insn_decode_kernel(&insn, addr);
    if (WARN_ON_ONCE(ret < 0))
    continue;

    - op1 = insn.opcode.bytes[0];
    - if (WARN_ON_ONCE(op1 != JMP32_INSN_OPCODE))
    + op = insn.opcode.bytes[0];
    + if (op == JMP32_INSN_OPCODE)
    + dest = addr + insn.length + insn.immediate.value;
    +
    + if (__static_call_fixup(addr, op, dest) ||
    + WARN_ON_ONCE(dest != &__x86_return_thunk))
    continue;

    DPRINTK("return thunk at: %pS (%px) len: %d to: %pS",
    --- a/arch/x86/kernel/static_call.c
    +++ b/arch/x86/kernel/static_call.c
    @@ -12,6 +12,13 @@ enum insn_type {
    };

    /*
    + * ud1 %esp, %ecx - a 3 byte #UD that is unique to trampolines, chosen such
    + * that there is no false-positive trampoline identification while also being a
    + * speculation stop.
    + */
    +static const u8 tramp_ud[] = { 0x0f, 0xb9, 0xcc };
    +
    +/*
    * cs cs cs xorl %eax, %eax - a single 5 byte instruction that clears %[er]ax
    */
    static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 };
    @@ -43,7 +50,10 @@ static void __ref __static_call_transfor
    break;

    case RET:
    - code = &retinsn;
    + if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
    + code = text_gen_insn(JMP32_INSN_OPCODE, insn, &__x86_return_thunk);
    + else
    + code = &retinsn;
    break;
    }

    @@ -109,3 +119,29 @@ void arch_static_call_transform(void *si
    mutex_unlock(&text_mutex);
    }
    EXPORT_SYMBOL_GPL(arch_static_call_transform);
    +
    +#ifdef CONFIG_RETPOLINE
    +/*
    + * This is called by apply_returns() to fix up static call trampolines,
    + * specifically ARCH_DEFINE_STATIC_CALL_NULL_TRAMP which is recorded as
    + * having a return trampoline.
    + *
    + * The problem is that static_call() is available before determining
    + * X86_FEATURE_RETHUNK and, by implication, running alternatives.
    + *
    + * This means that __static_call_transform() above can have overwritten the
    + * return trampoline and we now need to fix things up to be consistent.
    + */
    +bool __static_call_fixup(void *tramp, u8 op, void *dest)
    +{
    + if (memcmp(tramp+5, tramp_ud, 3)) {
    + /* Not a trampoline site, not our problem. */
    + return false;
    + }
    +
    + if (op == RET_INSN_OPCODE || dest == &__x86_return_thunk)
    + __static_call_transform(tramp, RET, NULL);
    +
    + return true;
    +}
    +#endif

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