In the Linux kernel, the following vulnerability has been resolved:

hugetlbfs: fix null-ptr-deref in hugetlbfsparseparam()

Syzkaller reports a null-ptr-deref bug as follows:

KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] RIP: 0010:hugetlbfsparseparam+0x1dd/0x8e0 fs/hugetlbfs/inode.c:1380 [...] Call Trace: <TASK> vfsparsefsparam fs/fscontext.c:148 [inline] vfsparsefsparam+0x1f9/0x3c0 fs/fscontext.c:129 vfsparsefsstring+0xdb/0x170 fs/fscontext.c:191 genericparsemonolithic+0x16f/0x1f0 fs/fscontext.c:231 donewmount fs/namespace.c:3036 [inline] pathmount+0x12de/0x1e20 fs/namespace.c:3370 do_mount fs/namespace.c:3383 [inline] __dosysmount fs/namespace.c:3591 [inline] __sesysmount fs/namespace.c:3568 [inline] __x64sysmount+0x27f/0x300 fs/namespace.c:3568 dosyscallx64 arch/x86/entry/common.c:50 [inline] dosyscall64+0x35/0xb0 arch/x86/entry/common.c:80 entrySYSCALL64afterhwframe+0x63/0xcd [...]

</TASK>

According to commit "vfs: parse: deal with zero length string value", kernel will set the param->string to null pointer in vfsparsefs_string() if fs string has zero length.

Yet the problem is that, hugetlbfsparseparam() will dereference the param->string, without checking whether it is a null pointer. To be more specific, if hugetlbfsparseparam() parses an illegal mount parameter, such as "size=,", kernel will constructs struct fsparameter with null pointer in vfsparsefsstring(), then passes this struct fsparameter to hugetlbfsparse_param(), which triggers the above null-ptr-deref bug.

This patch solves it by adding sanity check on param->string in hugetlbfsparseparam().