[PATCH] arm: explicitly place .fixup in .text

From: Nick Desaulniers
Date: Fri Nov 22 2019 - 13:55:30 EST

From: Kees Cook <keescook@xxxxxxxxxxxx>

There's an implicit dependency on the section ordering of the orphaned
section .fixup that can break arm_copy_from_user if the linker places
the .fixup section before the .text section. Since .fixup is not
explicitly placed in the existing ARM linker scripts, the linker is free
to order it anywhere with respect to the rest of the sections.

Multiple users from different distros (Raspbian, CrOS) reported kernel
panics executing seccomp() syscall with Linux kernels linked with LLD.

Documentation/x86/exception-tables.rst alludes to the ordering
dependency. The relevant quote:

Due to the way that the exception table is built and needs to be ordered,
only use exceptions for code in the .text section. Any other section
will cause the exception table to not be sorted correctly, and the
exceptions will fail.

Things changed when 64-bit support was added to x86 Linux. Rather than
double the size of the exception table by expanding the two entries
from 32-bits to 64 bits, a clever trick was used to store addresses
as relative offsets from the table itself. The assembly code changed

.long 1b,3b
.long (from) - .
.long (to) - .

and the C-code that uses these values converts back to absolute addresses
like this::

ex_insn_addr(const struct exception_table_entry *x)
return (unsigned long)&x->insn + x->insn;

Since the addresses stored in the __ex_table are RELATIVE offsets and
not ABSOLUTE addresses, ordering the fixup anywhere that's not
immediately preceding .text causes the relative offset of the faulting
instruction to be wrong, causing the wrong (or no) address of the fixup
handler to looked up in __ex_table.

x86 and arm64 place the .fixup section near the end of the .text
section; follow their pattern.

Cc: stable@xxxxxxxxxxxxxxx
Link: https://github.com/ClangBuiltLinux/linux/issues/282
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1020633#c36
Reported-by: Manoj Gupta <manojgupta@xxxxxxxxxx>
Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Debugged-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Worded-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Manoj Gupta <manojgupta@xxxxxxxxxx>
Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
arch/arm/kernel/vmlinux.lds.h | 1 +
1 file changed, 1 insertion(+)

diff --git a/arch/arm/kernel/vmlinux.lds.h b/arch/arm/kernel/vmlinux.lds.h
index 8247bc15addc..e130f7668cf0 100644
--- a/arch/arm/kernel/vmlinux.lds.h
+++ b/arch/arm/kernel/vmlinux.lds.h
@@ -74,6 +74,7 @@
+ *(.fixup) \
*(.gnu.warning) \
*(.glue_7) \
*(.glue_7t) \