[PATCH 1/1] lib: avoid bus error in fwts_safe_memcpy

[Heinrich Schuchardt]

The glibc 2.31 implementation of memcpy() may read 8 bytes at once, even if
this exceeds the source buffer. This can lead to a bus error.

Without this patch 'sudo fwts dmicheck -' results in an error when reading
the SMBIOS table on my system:

Test 1 of 4: Find and test SMBIOS Table Entry Points.
This test tries to find and sanity check the SMBIOS data structures.
Cannot read mmap'd SMBIOS entry at 0x0x7aef3000
SMBIOS entry loaded from /sys/firmware/dmi/tables/smbios_entry_point
FAILED [HIGH] SMBIOSNoEntryPoint: Test 1, Could not find any SMBIOS Table Entry Points.

Test 2 of 4: Test DMI/SMBIOS tables for errors.
SKIPPED: Test 2, Cannot find SMBIOS or DMI table entry, skip the test.

The reason is a bus error caught inside fwts_safe_memcpy():

Caught SIGNAL 7 (Bus error), aborting.
Backtrace:
0x0000ffff8e77d848 fwts/src/lib/src/.libs/libfwts.so.1(+0x16848)
0x0000ffff8e8fc7bc linux-vdso.so.1(__kernel_rt_sigreturn+0)
0x0000ffff8e6617ac /lib/aarch64-linux-gnu/libc.so.6(+0x887ac)
0x0000ffff8e7922a4 fwts/src/lib/src/.libs/libfwts.so.1(fwts_safe_memcpy+0x44)
0x0000ffff8e78f84c fwts/src/lib/src/.libs/libfwts.so.1(fwts_smbios_find_entry+0x6c)

The bus error is avoided by replacing memcpy() with a copy loop that
does not read outside the copied range:

PASSED: Test 1, SMBIOS Table Entry Point Checksum is valid.
PASSED: Test 1, SMBIOS Table Entry Point Length is valid.
PASSED: Test 1, SMBIOS Table Entry Intermediate Anchor String _DMI_ is valid.
PASSED: Test 1, SMBIOS Table Entry Point Intermediate Checksum is valid.
SMBIOS table loaded from /sys/firmware/dmi/tables/DMI
PASSED: Test 1, SMBIOS Table Entry Structure Table Address and Length looks valid.

Test 2 of 4: Test DMI/SMBIOS tables for errors.
SMBIOS table loaded from /sys/firmware/dmi/tables/DMI
PASSED: Test 2, Entry @ 0x7aef3020 'BIOS Information (Type 0)'
PASSED: Test 2, Entry @ 0x7aef3065 'System Information (Type 1)'
PASSED: Test 2, Entry @ 0x7aef30b2 'Base Board Information (Type 2)'
PASSED: Test 2, Entry @ 0x7aef30e0 'Chassis Information (Type 3)'

Signed-off-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
---
 src/lib/src/fwts_safe_mem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/lib/src/fwts_safe_mem.c b/src/lib/src/fwts_safe_mem.c
index 2121d73c..abe6500c 100644
--- a/src/lib/src/fwts_safe_mem.c
+++ b/src/lib/src/fwts_safe_mem.c
@@ -53,7 +53,8 @@ int OPTIMIZE0 fwts_safe_memcpy(void *dst, const void *src, const size_t n)

 	fwts_sig_handler_set(SIGSEGV, sig_handler, &old_segv_action);
 	fwts_sig_handler_set(SIGBUS, sig_handler, &old_bus_action);
-	memcpy(dst, src, n);
+	for (size_t i = n; i; --i)
+		*(char *)dst++ = *(char *)src++;
 	fwts_sig_handler_restore(SIGSEGV, &old_segv_action);
 	fwts_sig_handler_restore(SIGBUS, &old_bus_action);

--
2.28.0