[Bug 263822] malicious USB memory stick can cause panic in FFS taste code

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=263822

            Bug ID: 263822
           Summary: malicious USB memory stick can cause panic in FFS
                    taste code
           Product: Base System
           Version: Unspecified
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: rtm@lcs.mit.edu

Created attachment 233775
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=233775&action=edit
A disk image that causes a panic in ffs_sbget() during tasting.

If ffs_sbget() sees a superblock with fs_cssize and fs_ncg equal to
0xffffffff, the kernel panics because this UFS_MALLOC() is passed a
negative size:

        size = fs->fs_cssize;
        ...
        if (fs->fs_contigsumsize > 0)
                size += fs->fs_ncg * sizeof(int32_t);
        size += fs->fs_ncg * sizeof(u_int8_t);
        ...
        if ((space = UFS_MALLOC(size, filltype, M_WAITOK)) == NULL) {

The taste system calls ffs_sbget() for any newly attached storage
device. So a user can get this crash upon inserting a USB stick even
if they don't try to mount it as FFS.

This might be similar to or the same as
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=257557
but this report details the specific cause.

I've attached a demo disk image:

# uname -a
FreeBSD  14.0-CURRENT FreeBSD 14.0-CURRENT #202
main-n250917-440e36e68279-dirty: Fri May  6 10:56:11 EDT 2022    
rtm@xxx:/usr/obj/usr/rtm/symbsd/src/riscv.riscv64/sys/RTM  riscv
# mdconfig -f taste5b.img
panic: Assertion size > 0 failed at
/usr/rtm/symbsd/src/sys/kern/subr_vmem.c:1332                                   
cpuid = 0
time = 1651834366
KDB: stack backtrace:
db_trace_self() at db_trace_self
db_trace_self_wrapper() at db_trace_self_wrapper+0x38
kdb_backtrace() at kdb_backtrace+0x2c
vpanic() at vpanic+0x16e
panic() at panic+0x2a
vmem_alloc() at vmem_alloc+0xc8
kmem_malloc_domain() at kmem_malloc_domain+0x54
kmem_malloc_domainset() at kmem_malloc_domainset+0x36
malloc_large() at malloc_large+0x2a
malloc() at malloc+0xf8
ffs_sbget() at ffs_sbget+0x13c
g_label_ufs_taste_common() at g_label_ufs_taste_common+0x6c
g_label_ufs_id_taste() at g_label_ufs_id_taste+0xe
g_label_taste() at g_label_taste+0x198
g_new_provider_event() at g_new_provider_event+0xb8
one_event() at one_event+0x106
g_run_events() at g_run_events+0x8a
g_event_procbody() at g_event_procbody+0x56
fork_exit() at fork_exit+0x80
fork_trampoline() at fork_trampoline+0xa
KDB: enter: panic
[ thread pid 13 tid 100017 ]
Stopped at      breakpoint+0xa: c.ldsp  s0,0(sp)
db>

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