[PATCH] Discourage people from fiddling with kernel data from prom_init

[PATCH] Discourage people from fiddling with kernel data from prom_init

[Michael Ellerman]

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As BenH said the other day, it is an "accident" that prom_init.o is linked
with the rest of the kernel. The truth is a little more subtle, prom_init
isn't truly bootloader, it does fiddle with kernel data in a few places.

What we can do is discourage people from adding new code that accesses
data outside of prom_init. And hence this patch, from the script:

 # This script checks prom_init.o to see what external symbols it
 # is using, if it finds symbols not in the whitelist it returns
 # an error. The point of this is to discourage people from
 # intentionally or accidentally adding new code to prom_init.c
 # which has side effects on other parts of the kernel.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
---
 arch/powerpc/kernel/Makefile           |    9 +++++
 arch/powerpc/kernel/prom_init_check.sh |   58 ++++++++++++++++++++++++++++++++
 2 files changed, 67 insertions(+), 0 deletions(-)

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 5183a90..562bb02 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -106,4 +106,13 @@ PHONY += systbl_chk
 systbl_chk: $(src)/systbl_chk.sh $(obj)/systbl_chk.i
 	$(call cmd,systbl_chk)
 
+$(obj)/built-in.o:		prom_init_check
+
+quiet_cmd_prom_init_check = CALL    $<
+      cmd_prom_init_check = $(CONFIG_SHELL) $< "$(NM)" "$(obj)/prom_init.o"
+
+PHONY += prom_init_check
+prom_init_check: $(src)/prom_init_check.sh $(obj)/prom_init.o
+	$(call cmd,prom_init_check)
+
 clean-files := vmlinux.lds
diff --git a/arch/powerpc/kernel/prom_init_check.sh b/arch/powerpc/kernel/prom_init_check.sh
new file mode 100644
index 0000000..8e24fc1
--- /dev/null
+++ b/arch/powerpc/kernel/prom_init_check.sh
@@ -0,0 +1,58 @@
+#!/bin/sh
+#
+# Copyright © 2008 IBM Corporation
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version
+# 2 of the License, or (at your option) any later version.
+
+# This script checks prom_init.o to see what external symbols it
+# is using, if it finds symbols not in the whitelist it returns
+# an error. The point of this is to discourage people from
+# intentionally or accidentally adding new code to prom_init.c
+# which has side effects on other parts of the kernel.
+
+# If you really need to reference something from prom_init.o add
+# it to the list below:
+
+WHITELIST="add_reloc_offset __bss_start __bss_stop copy_and_flush
+_end enter_prom memcpy memset reloc_offset __secondary_hold
+__secondary_hold_acknowledge __secondary_hold_spinloop __start
+strcmp strcpy strlcpy strlen strncmp strstr logo_linux_clut224
+reloc_got2"
+
+NM="$1"
+OBJ="$2"
+
+ERROR=0
+
+for UNDEF in $($NM -u $OBJ | awk '{print $2}')
+do
+	# On 64-bit nm gives us the function descriptors, which have
+	# a leading . on the name, so strip it off here.
+	UNDEF="${UNDEF#.}"
+
+	if [ $KBUILD_VERBOSE ]; then
+		if [ $KBUILD_VERBOSE -ne 0 ]; then
+			echo "Checking prom_init.o symbol '$UNDEF'"
+		fi
+	fi
+
+	OK=0
+	for WHITE in $WHITELIST
+	do
+		if [ "$UNDEF" = "$WHITE" ]; then
+			OK=1
+			break
+		fi
+	done
+
+	if [ $OK -eq 0 ]; then
+		ERROR=1
+		echo "Error: External symbol '$UNDEF' referenced" \
+		     "from prom_init.c" >&2
+	fi
+done
+
+exit $ERROR
-- 
1.5.5

Re: [PATCH] Discourage people from fiddling with kernel data from prom_init

[Benjamin Herrenschmidt]

On Thu, 2008-04-24 at 12:08 +1000, Michael Ellerman wrote:
> As BenH said the other day, it is an "accident" that prom_init.o is linked
> with the rest of the kernel. The truth is a little more subtle, prom_init
> isn't truly bootloader, it does fiddle with kernel data in a few places.
> 
> What we can do is discourage people from adding new code that accesses
> data outside of prom_init. And hence this patch, from the script:
> 
>  # This script checks prom_init.o to see what external symbols it
>  # is using, if it finds symbols not in the whitelist it returns
>  # an error. The point of this is to discourage people from
>  # intentionally or accidentally adding new code to prom_init.c
>  # which has side effects on other parts of the kernel.
> 
> Signed-off-by: Michael Ellerman <michael@ellerman.id.au>

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

> ---
>  arch/powerpc/kernel/Makefile           |    9 +++++
>  arch/powerpc/kernel/prom_init_check.sh |   58 ++++++++++++++++++++++++++++++++
>  2 files changed, 67 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
> index 5183a90..562bb02 100644
> --- a/arch/powerpc/kernel/Makefile
> +++ b/arch/powerpc/kernel/Makefile
> @@ -106,4 +106,13 @@ PHONY += systbl_chk
>  systbl_chk: $(src)/systbl_chk.sh $(obj)/systbl_chk.i
>  	$(call cmd,systbl_chk)
>  
> +$(obj)/built-in.o:		prom_init_check
> +
> +quiet_cmd_prom_init_check = CALL    $<
> +      cmd_prom_init_check = $(CONFIG_SHELL) $< "$(NM)" "$(obj)/prom_init.o"
> +
> +PHONY += prom_init_check
> +prom_init_check: $(src)/prom_init_check.sh $(obj)/prom_init.o
> +	$(call cmd,prom_init_check)
> +
>  clean-files := vmlinux.lds
> diff --git a/arch/powerpc/kernel/prom_init_check.sh b/arch/powerpc/kernel/prom_init_check.sh
> new file mode 100644
> index 0000000..8e24fc1
> --- /dev/null
> +++ b/arch/powerpc/kernel/prom_init_check.sh
> @@ -0,0 +1,58 @@
> +#!/bin/sh
> +#
> +# Copyright © 2008 IBM Corporation
> +#
> +# This program is free software; you can redistribute it and/or
> +# modify it under the terms of the GNU General Public License
> +# as published by the Free Software Foundation; either version
> +# 2 of the License, or (at your option) any later version.
> +
> +# This script checks prom_init.o to see what external symbols it
> +# is using, if it finds symbols not in the whitelist it returns
> +# an error. The point of this is to discourage people from
> +# intentionally or accidentally adding new code to prom_init.c
> +# which has side effects on other parts of the kernel.
> +
> +# If you really need to reference something from prom_init.o add
> +# it to the list below:
> +
> +WHITELIST="add_reloc_offset __bss_start __bss_stop copy_and_flush
> +_end enter_prom memcpy memset reloc_offset __secondary_hold
> +__secondary_hold_acknowledge __secondary_hold_spinloop __start
> +strcmp strcpy strlcpy strlen strncmp strstr logo_linux_clut224
> +reloc_got2"
> +
> +NM="$1"
> +OBJ="$2"
> +
> +ERROR=0
> +
> +for UNDEF in $($NM -u $OBJ | awk '{print $2}')
> +do
> +	# On 64-bit nm gives us the function descriptors, which have
> +	# a leading . on the name, so strip it off here.
> +	UNDEF="${UNDEF#.}"
> +
> +	if [ $KBUILD_VERBOSE ]; then
> +		if [ $KBUILD_VERBOSE -ne 0 ]; then
> +			echo "Checking prom_init.o symbol '$UNDEF'"
> +		fi
> +	fi
> +
> +	OK=0
> +	for WHITE in $WHITELIST
> +	do
> +		if [ "$UNDEF" = "$WHITE" ]; then
> +			OK=1
> +			break
> +		fi
> +	done
> +
> +	if [ $OK -eq 0 ]; then
> +		ERROR=1
> +		echo "Error: External symbol '$UNDEF' referenced" \
> +		     "from prom_init.c" >&2
> +	fi
> +done
> +
> +exit $ERROR
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