kheaders: Use array declaration instead of char

Under CONFIG_FORTIFY_SOURCE, memcpy() will check the size of destination and source buffers. Defining kernel_headers_data as "char" would trip this check. Since these addresses are treated as byte arrays, define them as arrays (as done everywhere else).

This was seen with:

  $ cat /sys/kernel/kheaders.tar.xz >> /dev/null

detected buffer overflow in memcpy
kernel BUG at lib/string_helpers.c:1027!
...

  RIP: 0010:fortify_panic+0xf/0x20
  [...]
  Call Trace:
   <TASK>
   ikheaders_read+0x45/0x50 [kheaders]
   kernfs_fop_read_iter+0x1a4/0x2f0
  ...

Reported-by Jakub Kicinski <[email protected]>

Link https://lore.kernel.org/bpf/[email protected]/ open here

Tested-by Jakub Kicinski <[email protected]>

Fixes 43d8ce9d65a5 ("Provide in-kernel headers to make extending kernel easier")

Cc Joel Fernandes (Google) <[email protected]>

Cc [email protected]

Signed-off-by Kees Cook <[email protected]>

---
 kernel/kheaders.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

kernel/kheaders.c +5 -5

index 8f69772af77b..42163c9e94e5 100644
@@ -26,15 +26,15 @@ asm (
 "	.popsection				\n"
 );
 
-extern char kernel_headers_data;
-extern char kernel_headers_data_end;
+extern char kernel_headers_data[];
+extern char kernel_headers_data_end[];
 
 static ssize_t
 ikheaders_read(struct file *file,  struct kobject *kobj,
 	       struct bin_attribute *bin_attr,
 	       char *buf, loff_t off, size_t len)
 {
-	memcpy(buf, &kernel_headers_data + off, len);
+	memcpy(buf, &kernel_headers_data[off], len);
 	return len;
 }
 

@@ -48,8 +48,8 @@ static struct bin_attribute kheaders_attr __ro_after_init = {
 
 static int __init ikheaders_init(void)
 {
-	kheaders_attr.size = (&kernel_headers_data_end -
-			      &kernel_headers_data);
+	kheaders_attr.size = (kernel_headers_data_end -
+			      kernel_headers_data);
 	return sysfs_create_bin_file(kernel_kobj, &kheaders_attr);
 }
 

Signature · 1 line

2.34.1

Under CONFIG_FORTIFY_SOURCE, memcpy() will check the size of destination and source buffers. Defining kernel_headers_data as "char" would trip this check. Since these addresses are treated as byte arrays, define them as arrays (as done everywhere else).

This was seen with:

  $ cat /sys/kernel/kheaders.tar.xz >> /dev/null

detected buffer overflow in memcpy
kernel BUG at lib/string_helpers.c:1027!
...

  RIP: 0010:fortify_panic+0xf/0x20
  [...]
  Call Trace:
   <TASK>
   ikheaders_read+0x45/0x50 [kheaders]
   kernfs_fop_read_iter+0x1a4/0x2f0
  ...

Reported-by Jakub Kicinski <[email protected]>

Link https://lore.kernel.org/bpf/[email protected]/ open here

Tested-by Jakub Kicinski <[email protected]>

Fixes 43d8ce9d65a5 ("Provide in-kernel headers to make extending kernel easier")

Cc Joel Fernandes (Google) <[email protected]>

Cc [email protected]

Signed-off-by Kees Cook <[email protected]>

---
 kernel/kheaders.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

From: Kees Cook <[email protected]>
Date: Thu, 2 Mar 2023 14:49:50 -0800

Quoted 4 lines

Under CONFIG_FORTIFY_SOURCE, memcpy() will check the size of destination and source buffers. Defining kernel_headers_data as "char" would trip this check. Since these addresses are treated as byte arrays, define them as arrays (as done everywhere else).

Yet another array-as-one-char, I wonder how many are still here...

Quoted 9 lines

From: Kees Cook <[email protected]>
Date: Thu, 2 Mar 2023 14:49:50 -0800

Quoted 4 lines

Under CONFIG_FORTIFY_SOURCE, memcpy() will check the size of destination and source buffers. Defining kernel_headers_data as "char" would trip this check. Since these addresses are treated as byte arrays, define them as arrays (as done everywhere else).

Yet another array-as-one-char, I wonder how many are still here...

Yeah, good point. They do tend to stand out; we could find them:

$ git grep 'extern char [^\*\[\( ]*;'
arch/alpha/boot/bootp.c:extern char _end;
arch/alpha/boot/bootpz.c:extern char _end;
arch/alpha/boot/main.c: extern char _end;
arch/arm/mach-rockchip/core.h:extern char rockchip_secondary_trampoline;
arch/arm/mach-rockchip/core.h:extern char rockchip_secondary_trampoline_end;
arch/arm/mach-zynq/common.h:extern char zynq_secondary_trampoline;
arch/arm/mach-zynq/common.h:extern char zynq_secondary_trampoline_jump;
arch/arm/mach-zynq/common.h:extern char zynq_secondary_trampoline_end;
arch/hexagon/include/uapi/asm/setup.h:extern char external_cmdline_buffer;
arch/ia64/include/asm/smp.h:extern char no_int_routing;
arch/ia64/kernel/process.c:     extern char ia64_ret_from_clone;
arch/mips/dec/prom/memory.c:    extern char genexcept_early;
arch/mips/kernel/traps.c:       extern char except_vec3_generic;
arch/mips/kernel/traps.c:       extern char except_vec4;
arch/mips/kernel/traps.c:       extern char except_vec3_r4000;
arch/mips/mm/c-octeon.c:        extern char except_vec2_octeon;
arch/parisc/boot/compressed/misc.c:extern char output_len;
arch/parisc/boot/compressed/misc.c:extern char _startcode_end;
arch/powerpc/include/asm/smp.h:extern char __secondary_hold;
arch/s390/include/asm/kvm_host.h:extern char sie_exit;
arch/sh/boards/mach-ap325rxa/setup.c:extern char ap325rxa_sdram_enter_start;
arch/sh/boards/mach-ap325rxa/setup.c:extern char ap325rxa_sdram_enter_end;
arch/sh/boards/mach-ap325rxa/setup.c:extern char ap325rxa_sdram_leave_start;
arch/sh/boards/mach-ap325rxa/setup.c:extern char ap325rxa_sdram_leave_end;
arch/sh/boards/mach-ecovec24/setup.c:extern char ecovec24_sdram_enter_start;
arch/sh/boards/mach-ecovec24/setup.c:extern char ecovec24_sdram_enter_end;
arch/sh/boards/mach-ecovec24/setup.c:extern char ecovec24_sdram_leave_start;
arch/sh/boards/mach-ecovec24/setup.c:extern char ecovec24_sdram_leave_end;
arch/sh/boards/mach-kfr2r09/setup.c:extern char kfr2r09_sdram_enter_start;
arch/sh/boards/mach-kfr2r09/setup.c:extern char kfr2r09_sdram_enter_end;
arch/sh/boards/mach-kfr2r09/setup.c:extern char kfr2r09_sdram_leave_start;
arch/sh/boards/mach-kfr2r09/setup.c:extern char kfr2r09_sdram_leave_end;
arch/sh/boards/mach-migor/setup.c:extern char migor_sdram_enter_start;
arch/sh/boards/mach-migor/setup.c:extern char migor_sdram_enter_end;
arch/sh/boards/mach-migor/setup.c:extern char migor_sdram_leave_start;
arch/sh/boards/mach-migor/setup.c:extern char migor_sdram_leave_end;
arch/sh/boards/mach-se/7724/setup.c:extern char ms7724se_sdram_enter_start;
arch/sh/boards/mach-se/7724/setup.c:extern char ms7724se_sdram_enter_end;
arch/sh/boards/mach-se/7724/setup.c:extern char ms7724se_sdram_leave_start;
arch/sh/boards/mach-se/7724/setup.c:extern char ms7724se_sdram_leave_end;
arch/sh/kernel/cpu/shmobile/pm.c:extern char sh_mobile_sleep_enter_start;
arch/sh/kernel/cpu/shmobile/pm.c:extern char sh_mobile_sleep_enter_end;
arch/sh/kernel/cpu/shmobile/pm.c:extern char sh_mobile_sleep_resume_start;
arch/sh/kernel/cpu/shmobile/pm.c:extern char sh_mobile_sleep_resume_end;
arch/x86/entry/vsyscall/vsyscall_64.c:  extern char __vsyscall_page;
arch/x86/include/asm/vvar.h:extern char __vvar_page;
kernel/configs.c:extern char kernel_config_data;
kernel/configs.c:extern char kernel_config_data_end;
net/bpfilter/bpfilter_kern.c:extern char bpfilter_umh_start;
net/bpfilter/bpfilter_kern.c:extern char bpfilter_umh_end;
samples/bpf/task_fd_query_user.c:       extern char __executable_start;
tools/testing/selftests/kvm/lib/aarch64/processor.c:    extern char vectors;
tools/testing/selftests/x86/test_syscall_vdso.c:extern char int80;

Of those, it looks like only a handful might trip FORTIFY:

$ for i in $(git grep 'extern char [^\*\[\( ]*;' | grep -v boot/ | awk -F' ' '{print $NF}' | cut -d';' -f1); do git grep -E '(strcpy|memcpy|memset).*'"$i",; done
arch/arm/mach-rockchip/platsmp.c:       memcpy_toio(sram_base_addr, &rockchip_secondary_trampoline, trampoline_sz);
arch/arm/mach-zynq/platsmp.c:                   memcpy_toio(zero, &zynq_secondary_trampoline,
arch/mips/dec/prom/memory.c:    memcpy((void *)(CKSEG0 + 0x80), &genexcept_early, 0x80);
arch/sh/kernel/cpu/shmobile/pm.c:       memcpy(vp, &sh_mobile_sleep_enter_start, n);
arch/sh/kernel/cpu/shmobile/pm.c:       memcpy(vp, &sh_mobile_sleep_resume_start, n);
arch/arm64/mm/trans_pgd.c:      memcpy(hyp_stub, &trans_pgd_stub_vectors, ARM64_VECTOR_TABLE_LEN);