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Message-Id: <20230711085837.fac80c964ea7667cb75bd6e5@kernel.org>
Date: Tue, 11 Jul 2023 08:58:37 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Petr Pavlu <petr.pavlu@...e.com>, tglx@...utronix.de,
mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
hpa@...or.com, samitolvanen@...gle.com, x86@...nel.org,
linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] x86/kprobes: Prohibit probing on compiler
generated CFI checking code
On Mon, 10 Jul 2023 18:16:43 +0200
Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Jul 10, 2023 at 09:14:35PM +0900, Masami Hiramatsu (Google) wrote:
> > + if (IS_ENABLED(CONFIG_CFI_CLANG)) {
> > + /*
> > + * The compiler generates the following instruction sequence
> > + * for indirect call checks and cfi.c decodes this;
> > + *
> > + * movl -<id>, %r10d ; 6 bytes
> > + * addl -4(%reg), %r10d ; 4 bytes
> > + * je .Ltmp1 ; 2 bytes
> > + * ud2 ; <- regs->ip
> > + * .Ltmp1:
> > + *
> > + * Also, these movl and addl are used for showing expected
> > + * type. So those must not be touched.
> > + */
> > + __addr = recover_probed_instruction(buf, addr);
> > + if (!__addr)
> > + return 0;
> > +
> > + if (insn_decode_kernel(&insn, (void *)__addr) < 0)
> > + return 0;
> > +
> > + if (insn.opcode.value == 0xBA)
> > + offset = 12;
> > + else if (insn.opcode.value == 0x3)
> > + offset = 6;
> > + else
> > + goto out;
>
> Notably, the JE will already be avoided?
>
> > +
> > + /* This movl/addl is used for decoding CFI. */
> > + if (is_cfi_trap(addr + offset))
> > + return 0;
> > + }
> >
> > +out:
> > return (addr == paddr);
> > }
>
> Hmm, so I was thinking something like the below, which also catches
> things when we rewrite kCFI to FineIBT, except I don't think we care if
> the FineIBT callsite gets re-written. FineIBT only relies on the __cfi_
> symbol not getting poked at (which the previous patches should ensure).
Oh, is FineIBT different from kCFI? I thought those are same. But anyway
for kCFI=y && FineIBT=n case, I think this code still needed.
>
> Additionally is_cfi_trap() is horrifically slow -- it's a full linear
> search of the entire kcfi_traps array, it doesn't have any smarts on
> (#UD can hardly be considered a fast path).
Yeah, register_kprobe() is not a fast path in most cases. So I think
this is OK at this point. We can speed it up by sorting the array by
address and binary search later.
>
> So I tihnk I'm ok with the above, just adding the below for reference
> (completely untested and everything).
I wonder the distance can be used outside of x86. CFI implementation
depends on the arch?
>
>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Thanks!
>
>
> ---
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index f7f6042eb7e6..b812dee76909 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -293,6 +293,11 @@ static int can_probe(unsigned long paddr)
> #endif
> addr += insn.length;
> }
> + /*
> + * Don't allow poking the kCFI/FineIBT callsites.
> + */
> + if (IS_ENABLED(CONFIG_CFI_CLANG) && cfi_call_site(addr))
> + return 0;
>
> return (addr == paddr);
> }
> diff --git a/kernel/cfi.c b/kernel/cfi.c
> index 08caad776717..2656e6ffa013 100644
> --- a/kernel/cfi.c
> +++ b/kernel/cfi.c
> @@ -31,16 +31,22 @@ static inline unsigned long trap_address(s32 *p)
> return (unsigned long)((long)p + (long)*p);
> }
>
> -static bool is_trap(unsigned long addr, s32 *start, s32 *end)
> +static long cfi_trap_distance(unsigned long addr, s32 *start, s32 *end)
> {
> + long dist = LONG_MAX;
> s32 *p;
>
> for (p = start; p < end; ++p) {
> - if (trap_address(p) == addr)
> - return true;
> + long d = trap_address(p) - addr;
> +
> + if (abs(dist) < abs(d)) {
> + dist = d;
> + if (dist == 0)
> + return 0;
> + }
> }
>
> - return false;
> + return dist;
> }
>
> #ifdef CONFIG_MODULES
> @@ -66,25 +72,25 @@ void module_cfi_finalize(const Elf_Ehdr *hdr, const Elf_Shdr *sechdrs,
> }
> }
>
> -static bool is_module_cfi_trap(unsigned long addr)
> +static long module_cfi_trap_distance(unsigned long addr)
> {
> struct module *mod;
> - bool found = false;
> + long dist = LONG_MAX;
>
> rcu_read_lock_sched_notrace();
>
> mod = __module_address(addr);
> if (mod)
> - found = is_trap(addr, mod->kcfi_traps, mod->kcfi_traps_end);
> + dist = cfi_trap_distance(addr, mod->kcfi_traps, mod->kcfi_traps_end);
>
> rcu_read_unlock_sched_notrace();
>
> return found;
> }
> #else /* CONFIG_MODULES */
> -static inline bool is_module_cfi_trap(unsigned long addr)
> +static long module_cfi_trap_distance(unsigned long addr)
> {
> - return false;
> + return LONG_MAX;
> }
> #endif /* CONFIG_MODULES */
>
> @@ -93,9 +99,24 @@ extern s32 __stop___kcfi_traps[];
>
> bool is_cfi_trap(unsigned long addr)
> {
> - if (is_trap(addr, __start___kcfi_traps, __stop___kcfi_traps))
> + long dist = cfi_trap_distance(addr, __start___kcfi_traps, __stop___kcfi_traps);
> + if (!dist)
> + return true;
> +
> + return module_cfi_trap_distance(addr) == 0;
> +}
> +
> +bool cfi_call_site(unsigned long addr)
> +{
> + long dist = cfi_trap_distance(addr, __start___kcfi_traps, __stop___kcfi_traps);
> + if (dist >= -12 && dist <= 0)
> + return true;
> +
> + dist = module_cfi_trap_distance(addr);
> + if (dist >= -12 && dist <= 0)
> return true;
>
> - return is_module_cfi_trap(addr);
> + return false;
> }
> +
> #endif /* CONFIG_ARCH_USES_CFI_TRAPS */
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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