Re: [PATCH v3] libfdt: fdt_get_alias_namelen: Validate aliases

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Oct 10, 2023 at 10:27:25AM +0100, Pierre-Clément Tosi wrote:
> Ensure that the alias found matches the device tree specification v0.4:
> 
>     Each property of the /aliases node defines an alias. The property
>     name specifies the alias name. The property value specifies the full
>     path to a node in the devicetree.
> 
> This protects against a stack overflow caused by
> 
>     fdt_path_offset_namelen(fdt, path, namelen)
> 
> calling
> 
>     fdt_path_offset(fdt, fdt_get_alias_namelen(fdt, path, namelen))
> 
> leading to infinite recursion on DTs with "circular" aliases.
> 
> This fix was originally written by Mike McTernan for Android in [1].
> 
> [1]: https://android.googlesource.com/platform/external/dtc/+/9308e7f9772bd226fea9925b1fc4d53c127ed4d5
> 
> Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>

Thanks, applied, with Mike McTernan's ack folded in.

> 
> ---
> v2
>   - replace memchr('/') with check on last character
>   - add test coverage of a self-referencing alias
>   - drop redundant test case on alias to non-absolute path
>   - reference the DT spec and AOSP patch in the commit message
>   - rephrase the infinite recursion case in the commit message
> v3
>   - swap (len > 0) and (alias) checks to better align with the API
>   - point the "relative" alias to a path that exists in the test DT
>   - simplify the infinite recursion case in the commit message
> ---
>  libfdt/fdt_ro.c   | 11 ++++++++++-
>  tests/aliases.dts |  4 ++++
>  tests/get_alias.c | 14 +++++++++++++-
>  3 files changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/libfdt/fdt_ro.c b/libfdt/fdt_ro.c
> index c4c520c..9fe2f49 100644
> --- a/libfdt/fdt_ro.c
> +++ b/libfdt/fdt_ro.c
> @@ -537,7 +537,16 @@ static const void *fdt_path_getprop_namelen(const void *fdt, const char *path,
>  const char *fdt_get_alias_namelen(const void *fdt,
>  				  const char *name, int namelen)
>  {
> -	return fdt_path_getprop_namelen(fdt, "/aliases", name, namelen, NULL);
> +	int len;
> +	const char *alias;
> +
> +	alias = fdt_path_getprop_namelen(fdt, "/aliases", name, namelen, &len);
> +
> +	if (!can_assume(VALID_DTB) &&
> +	    !(alias && len > 0 && alias[len - 1] == '\0' && *alias == '/'))
> +		return NULL;
> +
> +	return alias;
>  }
>  
>  const char *fdt_get_alias(const void *fdt, const char *name)
> diff --git a/tests/aliases.dts b/tests/aliases.dts
> index 853479a..03ed675 100644
> --- a/tests/aliases.dts
> +++ b/tests/aliases.dts
> @@ -5,6 +5,10 @@
>  	#size-cells = <0>;
>  
>  	aliases {
> +		empty = "";
> +		loop = "loop";
> +		nonull = [626164];
> +		relative = "s1/subsubnode";
>  		s1 = &sub1;
>  		ss1 = &subsub1;
>  		sss1 = &subsubsub1;
> diff --git a/tests/get_alias.c b/tests/get_alias.c
> index fb2c38c..d2888d6 100644
> --- a/tests/get_alias.c
> +++ b/tests/get_alias.c
> @@ -21,9 +21,16 @@ static void check_alias(void *fdt, const char *path, const char *alias)
>  
>  	aliaspath = fdt_get_alias(fdt, alias);
>  
> -	if (path && !aliaspath)
> +	if (!path && !aliaspath)
> +		return;
> +
> +	if (!aliaspath)
>  		FAIL("fdt_get_alias(%s) failed\n", alias);
>  
> +	if (!path)
> +		FAIL("fdt_get_alias(%s) returned %s instead of NULL",
> +		     alias, aliaspath);
> +
>  	if (strcmp(aliaspath, path) != 0)
>  		FAIL("fdt_get_alias(%s) returned %s instead of %s\n",
>  		     alias, aliaspath, path);
> @@ -36,9 +43,14 @@ int main(int argc, char *argv[])
>  	test_init(argc, argv);
>  	fdt = load_blob_arg(argc, argv);
>  
> +	check_alias(fdt, NULL, "empty");
> +	check_alias(fdt, NULL, "nonull");
> +	check_alias(fdt, NULL, "relative");
>  	check_alias(fdt, "/subnode@1", "s1");
>  	check_alias(fdt, "/subnode@1/subsubnode", "ss1");
>  	check_alias(fdt, "/subnode@1/subsubnode/subsubsubnode", "sss1");
>  
> +	check_alias(fdt, NULL, "loop"); // Might trigger a stack overflow
> +
>  	PASS();
>  }

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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