[Bug 263971] malicious superblock can cause buffer overflow during tasting

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=263971

            Bug ID: 263971
           Summary: malicious superblock can cause buffer overflow during
                    tasting
           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 233909
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=233909&action=edit
disk image with malicious superblock fields that cause buffer overflow during
tasting

ffs_sbget() allocates some memory into which it reads file system
information, using fields from the superblock to choose a 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) {

An attacker can choose values that cause size to be too small, causing
subsequent code to write beyond the end of space.

The attached disk image has a superblock with

  fs_cssize -1434999068
  fs_contigsumsize 2147483647
  fs_ncg 287173615

The calculated size is 869007.

Later on:

    if (fs->fs_contigsumsize > 0) {
        fs->fs_maxcluster = lp = (int32_t *)space;
        for (i = 0; i < fs->fs_ncg; i++)
            *lp++ = fs->fs_contigsumsize;

Since fs_ncg > size, this loop writes the attacker-chosen
fs_contigsumsize beyond the allocated space.

What I see when the attached disk image is tasted:

# uname -a
FreeBSD  14.0-CURRENT FreeBSD 14.0-CURRENT #267
main-n250920-74195f0a7a6c-dirty: Sat May 14 09:04:48 EDT 2022    
rtm@xxx:/usr/obj/usr/rtm/symbsd/src/riscv.riscv64/sys/RTM  riscv
# mdconfig -f taste7a.img
panic: vm_fault_lookup: fault on nofault entry, addr: 0xffffffc07cb67000
cpuid = 0
time = 1652523702
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
vm_fault_lookup() at vm_fault_lookup+0x1bc
vm_fault() at vm_fault+0x9e
vm_fault_trap() at vm_fault_trap+0x68
page_fault_handler() at page_fault_handler+0x13c
do_trap_supervisor() at do_trap_supervisor+0x76
cpu_exception_handler_supervisor() at cpu_exception_handler_supervisor+0x70
--- exception 15, tval = 0xffffffc07cb67000
ffs_sbget() at ffs_sbget+0x2b0
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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