Affected: Linux

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

fs/9p: fix NULL pointer dereference on mkdir

When a 9p tree was mounted with option 'posixacl', parent directory had a default ACL set for its subdirectories, e.g.:

setfacl -m default:group:simpsons:rwx parentdir

then creating a subdirectory crashed 9p client, as v9fs_fid_add() call in function v9fs_vfs_mkdir_dotl() sets the passed 'fid' pointer to NULL (since dafbe689736) even though the subsequent v9fs_set_create_acl() call expects a valid non-NULL 'fid' pointer:

[ 37.273191] BUG: kernel NULL pointer dereference, address: 0000000000000000 ... [ 37.322338] Call Trace: [ 37.323043] <TASK> [ 37.323621] ? __die (arch/x86/kernel/dumpstack.c:421 arch/x86/kernel/dumpstack.c:434) [ 37.324448] ? page_fault_oops (arch/x86/mm/fault.c:714) [ 37.325532] ? search_module_extables (kernel/module/main.c:3733) [ 37.326742] ? p9_client_walk (net/9p/client.c:1165) 9pnet [ 37.328006] ? search_bpf_extables (kernel/bpf/core.c:804) [ 37.329142] ? exc_page_fault (./arch/x86/include/asm/paravirt.h:686 arch/x86/mm/fault.c:1488 arch/x86/mm/fault.c:1538) [ 37.330196] ? asm_exc_page_fault (./arch/x86/include/asm/idtentry.h:574) [ 37.331330] ? p9_client_walk (net/9p/client.c:1165) 9pnet [ 37.332562] ? v9fs_fid_xattr_get (fs/9p/xattr.c:30) 9p [ 37.333824] v9fs_fid_xattr_set (fs/9p/fid.h:23 fs/9p/xattr.c:121) 9p [ 37.335077] v9fs_set_acl (fs/9p/acl.c:276) 9p [ 37.336112…