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The primary bug Alyssa noticed was that with FineIBT enabled function
prologues have a spurious ENDBR instruction:
__cfi_foo:
endbr64
subl $hash, %r10d
jz 1f
ud2
nop
1:
foo:
endbr64 <--- *sadface*
This means that any indirect call that fails to target the __cfi symbol
and instead targets (the regular old) foo+0, will succeed due to that
second ENDBR.
Fixing this lead to the discovery of a single indirect call that was
still doing this: ret_from_fork(), since that's an assembly stub the
compmiler would not generate the proper kCFI indirect call magic and it
would not get patched.
Brian came up with the most comprehensive fix -- convert the thing to C
with only a very thin asm wrapper. This ensures the kernel thread
boostrap is a proper kCFI call.
While discussing all this, Kees noted that kCFI hashes could/should be
poisoned to seal all functions whose address is never taken, further
limiting the valid kCFI targets -- much like we already do for IBT.
So what was a 'simple' observation and fix cascaded into a bunch of
inter-related CFI infrastructure fixes.
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Merge tag 'x86_urgent_for_6.5_rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull x86 CFI fixes from Peter Zijlstra:
"Fix kCFI/FineIBT weaknesses
The primary bug Alyssa noticed was that with FineIBT enabled function
prologues have a spurious ENDBR instruction:
__cfi_foo:
endbr64
subl $hash, %r10d
jz 1f
ud2
nop
1:
foo:
endbr64 <--- *sadface*
This means that any indirect call that fails to target the __cfi
symbol and instead targets (the regular old) foo+0, will succeed due
to that second ENDBR.
Fixing this led to the discovery of a single indirect call that was
still doing this: ret_from_fork(). Since that's an assembly stub the
compiler would not generate the proper kCFI indirect call magic and it
would not get patched.
Brian came up with the most comprehensive fix -- convert the thing to
C with only a very thin asm wrapper. This ensures the kernel thread
boostrap is a proper kCFI call.
While discussing all this, Kees noted that kCFI hashes could/should be
poisoned to seal all functions whose address is never taken, further
limiting the valid kCFI targets -- much like we already do for IBT.
So what was a 'simple' observation and fix cascaded into a bunch of
inter-related CFI infrastructure fixes"
* tag 'x86_urgent_for_6.5_rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
x86/cfi: Only define poison_cfi() if CONFIG_X86_KERNEL_IBT=y
x86/fineibt: Poison ENDBR at +0
x86: Rewrite ret_from_fork() in C
x86/32: Remove schedule_tail_wrapper()
x86/cfi: Extend ENDBR sealing to kCFI
x86/alternative: Rename apply_ibt_endbr()
x86/cfi: Extend {JMP,CAKK}_NOSPEC comment
b6e6cc1f78