https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=210864
Bug ID: 210864
Summary: ggatel(8) BIO_DELETE implementation is broken
Product: Base System
Version: 11.0-CURRENT
Hardware: Any
OS: Any
Status: New
Severity: Affects Only Me
Priority: ---
Component: bin
Assignee: [email protected]
Reporter: [email protected]
md(4) simulates BIO_DELETE by writing zeros, and I infer ggatel(8) is
trying to do the same based on the code structure similarity. However, the
latter's implementation is actually broken. If
g_gate_ioctl(G_GATE_CMD_START, ...) returns with a BIO_DELETE request, only
gctl_length and gctl_offset are updated, and gctl_data still points to the
original buffer. ggatel(8) then proceeds to service BIO_DELETE as if it
were BIO_WRITE, so it writes "garbage" to disk, and (hopefully) EFAULT if
gctl_length is greater than gctl_data's actual length.
PoC:
# cd /tmp
# clang -x c -o bug - <<'BUG'
#include <assert.h>
#include <fcntl.h>
#include <sys/disk.h>
#include <sys/ioctl.h>
int main()
{
const int fd = open("/dev/ggate0", O_RDWR | O_CLOEXEC | O_DIRECT);
assert(fd != -1);
off_t ol[2] = { 40960, 4096 };
assert(ioctl(fd, DIOCGDELETE, ol) != -1);
return(0);
}
BUG
# truncate -s 10m test
# ggatel create -s 4096 test
# dd if=/dev/zero bs=4096 count=1 | LC_ALL=C tr '\0' '\252' |
dd of=/dev/ggate0 bs=4096
# ./bug
# hexdump -C /dev/ggate0
00000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa |................|
*
00001000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
0000a000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa |................|
*
0000b000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00a00000
I'm not sure of the security implication of this bug, since to trigger the
bug directly requires elevated privileges. However, there may be clever
ways to trigger it by proxy.
IMHO, simulating BIO_DELETE by writing zeros is questionable, however not
everyone agrees[0]. That said, you can always use md(4) instead, so I
suggest ggatel(8) should just return EOPNOTSUPP for BIO_DELETE.
[0] http://thread.gmane.org/gmane.os.freebsd.devel.hackers/57688/focus=57689
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