A while ago [0] FineIBT started using the 0xEA instruction to raise #UD.
All existing parts will generate #UD in 64bit mode on that instruction.
However; Intel/AMD have not blessed using this instruction, it is on
their 'reserved' opcode list for future use.
Peter Anvin worked the committees and got use of 0xD6 blessed, it
shall be called UDB (per the next SDM or so), and it being a single
byte instruction is easy to slip into a single byte immediate -- as
is done by this very patch.
Reworking the FineIBT code to use UDB wasn't entirely trivial. Notably
the FineIBT-BHI1 case ran out of bytes. In order to condense the
encoding some it was required to move the hash register from R10D to
EAX (thanks hpa!).
Per the x86_64 ABI, RAX is used to pass the number of vector registers
for vararg function calls -- something that should not happen in the
kernel. More so, the kernel is built with -mskip-rax-setup, which
should leave RAX completely unused, allowing its re-use.
[ For BPF; while the bpf2bpf tail-call uses RAX in its calling
convention, that does not use CFI and is unaffected. Only the
'regular' C->BPF transition is covered by CFI. ]
The ENDBR poison value is changed from 'OSP NOP3' to 'NOPL -42(%RAX)',
this is basically NOP4 but with UDB as its immediate. As such it is
still a non-standard NOP value unique to prior ENDBR sites, but now
also provides UDB.
Per Agner Fog's optimization guide, Jcc is assumed not-taken. That is,
the expected path should be the fallthrough case for improved
throughput.
Since the preamble now relies on the ENDBR poison to provide UDB, the
code is changed to write the poison right along with the initial
preamble -- this is possible because the ITS mitigation already
disabled IBT over rewriting the CFI scheme.
The scheme in detail:
Preamble:
FineIBT FineIBT-BHI1 FineIBT-BHI
__cfi_\func: __cfi_\func: __cfi_\func:
endbr endbr endbr
subl $0x12345678, %eax subl $0x12345678, %eax subl $0x12345678, %eax
jne.d32,np \func+3 cmovne %rax, %rdi cs cs call __bhi_args_N
jne.d8,np \func+3
\func: \func: \func:
nopl -42(%rax) nopl -42(%rax) nopl -42(%rax)
Notably there are 7 bytes available after the SUBL; this enables the
BHI1 case to fit without the nasty overlapping case it had previously.
The !BHI case uses Jcc.d32,np to consume all 7 bytes without the need
for an additional NOP, while the BHI case uses CS padding to align the
CALL with the end of the preamble such that it returns to \func+0.
Caller:
FineIBT Paranoid-FineIBT
fineibt_caller: fineibt_caller:
mov $0x12345678, %eax mov $0x12345678, %eax
lea -10(%r11), %r11 cmp -0x11(%r11), %eax
nop5 cs lea -0x10(%r11), %r11
retpoline: retpoline:
cs call __x86_indirect_thunk_r11 jne fineibt_caller+0xd
call *%r11
nop
Notably this is before apply_retpolines() which will fix up the
retpoline call -- since all parts with IBT also have eIBRS (lets
ignore ITS). Typically the retpoline site is rewritten (when still
intact) into:
call *%r11
nop3
[0] 06926c6cdb ("x86/ibt: Optimize the FineIBT instruction sequence")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250901191307.GI4067720@noisy.programming.kicks-ass.net
While the GCC and Clang compilers already define __ASSEMBLER__
automatically when compiling assembly code, __ASSEMBLY__ is a
macro that only gets defined by the Makefiles in the kernel.
This can be very confusing when switching between userspace
and kernelspace coding, or when dealing with UAPI headers that
rather should use __ASSEMBLER__ instead. So let's standardize on
the __ASSEMBLER__ macro that is provided by the compilers now.
This is mostly a mechanical patch (done with a simple "sed -i"
statement), with some manual tweaks in <asm/frame.h>, <asm/hw_irq.h>
and <asm/setup.h> that mentioned this macro in comments with some
missing underscores.
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250314071013.1575167-38-thuth@redhat.com
Saves a CALL to an out-of-line thunk for the common case of 1
argument.
Suggested-by: Scott Constable <scott.d.constable@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20250224124200.927885784@infradead.org
Notably, don't attempt to decode an immediate when MOD == 3.
Additionally have it return the instruction length, such that WARN
like bugs can more reliably skip to the correct instruction.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20250207122546.721120726@infradead.org
Pretty much every caller of is_endbr() actually wants to test something at an
address and ends up doing get_kernel_nofault(). Fold the lot into a more
convenient helper.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20250207122546.181367417@infradead.org
The current name doesn't reflect what it does very well.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lkml.kernel.org/r/20230622144321.427441595%40infradead.org
UEFI v2.10 extends the EFI memory attributes table with a flag that
indicates whether or not all RuntimeServicesCode regions were
constructed with ENDBR landing pads, permitting the OS to map these
regions with IBT restrictions enabled.
So let's take this into account on x86 as well.
Suggested-by: Peter Zijlstra <peterz@infradead.org> # ibt_save() changes
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Add a macro which prevents a function from getting sealed if there are
no compile-time references to it.
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Message-Id: <20220818213927.e44fmxkoq4yj6ybn@treble>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Objtool's --ibt option generates .ibt_endbr_seal which lists
superfluous ENDBR instructions. That is those instructions for which
the function is never indirectly called.
Overwrite these ENDBR instructions with a NOP4 such that these
function can never be indirect called, reducing the number of viable
ENDBR targets in the kernel.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154319.822545231@infradead.org
Add Kconfig, Makefile and basic instruction support for x86 IBT.
(Ab)use __DISABLE_EXPORTS to disable IBT since it's already employed
to mark compressed and purgatory. Additionally mark realmode with it
as well to avoid inserting ENDBR instructions there. While ENDBR is
technically a NOP, inserting them was causing some grief due to code
growth. There's also a problem with using __noendbr in code compiled
without -fcf-protection=branch.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154317.519875203@infradead.org
