crypto: af_alg - avoid undefined behavior accessing salg_name

Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm names") made the kernel start accepting arbitrarily long algorithm names in sockaddr_alg. However, the actual length of the salg_name field stayed at the original 64 bytes. This is broken because the kernel can access indices >= 64 in salg_name, which is undefined behavior -- even though the memory that is accessed is still located within the sockaddr structure. It would only be defined behavior if the array were properly marked as arbitrary-length (either by making it a flexible array, which is the recommended way these days, or by making it an array of length 0 or 1). We can't simply change salg_name into a flexible array, since that would break source compatibility with userspace programs that embed sockaddr_alg into another struct, or (more commonly) declare a sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'. One solution would be to change salg_name into a flexible array only when '#ifdef __KERNEL__'. However, that would keep userspace without an easy way to actually use the longer algorithm names. Instead, add a new structure 'sockaddr_alg_new' that has the flexible array field, and expose it to both userspace and the kernel. Make the kernel use it correctly in alg_bind(). This addresses the syzbot report "UBSAN: array-index-out-of-bounds in alg_bind" (https://syzkaller.appspot.com/bug?extid=92ead4eb8e26a26d465e).

Eric Biggers committed 6 years ago

Commit 92eb6c31 parent 2d653932 files · +23 −3Browse files

Showing the changes to include/uapi (1 of 2 files). Show all

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include/uapi/linux/if_alg.h+16−0

@@ -24,6 +24,22 @@ struct sockaddr_alg { · 23 unmodified lines
24	__u8 salg_name[64];	24	__u8 salg_name[64];
25	};	25	};
26		26
		27	/*
		28	* Linux v4.12 and later removed the 64-byte limit on salg_name[]; it's now an
		29	* arbitrary-length field. We had to keep the original struct above for source
		30	* compatibility with existing userspace programs, though. Use the new struct
		31	* below if support for very long algorithm names is needed. To do this,
		32	* allocate 'sizeof(struct sockaddr_alg_new) + strlen(algname) + 1' bytes, and
		33	* copy algname (including the null terminator) into salg_name.
		34	*/
		35	struct sockaddr_alg_new {
		36	__u16 salg_family;
		37	__u8 salg_type[14];
		38	__u32 salg_feat;
		39	__u32 salg_mask;
		40	__u8 salg_name[];
		41	};
		42
27	struct af_alg_iv {	43	struct af_alg_iv {
28	__u32 ivlen;	44	__u32 ivlen;
29	__u8 iv[0];	45	__u8 iv[0];