On Tuesday, June 6, 2023 6:00:28 PM CEST Greg Kurz wrote: > Hi Christian, > > On Tue, 06 Jun 2023 15:57:50 +0200 > Christian Schoenebeck <qemu_...@crudebyte.com> wrote: > > > The 9p protocol does not specifically define how server shall behave when > > client tries to open a special file, however from security POV it does > > make sense for 9p server to prohibit opening any special file on host side > > in general. A sane Linux 9p client for instance would never attempt to > > open a special file on host side, it would always handle those exclusively > > on its guest side. A malicious client however could potentially escape > > from the exported 9p tree by creating and opening a device file on host > > side. > > > > With QEMU this could only be exploited in the following unsafe setups: > > > > - Running QEMU binary as root AND 9p 'local' fs driver AND 'passthrough' > > security model. > > > > or > > > > - Using 9p 'proxy' fs driver (which is running its helper daemon as > > root). > > > > These setups were already discouraged for safety reasons before, > > however for obvious reasons we are now tightening behaviour on this. > > > > Fixes: CVE-2023-2861 > > Reported-by: Yanwu Shen <yws...@gmail.com> > > Reported-by: Jietao Xiao <shawtao1...@gmail.com> > > Reported-by: Jinku Li <j...@xidian.edu.cn> > > Reported-by: Wenbo Shen <shenwe...@zju.edu.cn> > > Signed-off-by: Christian Schoenebeck <qemu_...@crudebyte.com> > > --- > > v1 -> v2: > > - Add equivalent fix for 'proxy' fs driver. > > - Minor adjustments on commit log. > > > > Note that this might be a bit confusing for reviewers since > v1 was never posted to qemu-devel. Technically, this should > have been posted without the v2 tag.
I felt it wouldn't make it any better, as it might otherwise confuse those who already got the previous two patch emails. > > fsdev/virtfs-proxy-helper.c | 48 +++++++++++++++++++++++++++++++++++-- > > hw/9pfs/9p-util.h | 29 ++++++++++++++++++++++ > > 2 files changed, 75 insertions(+), 2 deletions(-) > > > > diff --git a/fsdev/virtfs-proxy-helper.c b/fsdev/virtfs-proxy-helper.c > > index 5cafcd7703..f311519fa3 100644 > > --- a/fsdev/virtfs-proxy-helper.c > > +++ b/fsdev/virtfs-proxy-helper.c > > @@ -26,6 +26,7 @@ > > #include "qemu/xattr.h" > > #include "9p-iov-marshal.h" > > #include "hw/9pfs/9p-proxy.h" > > +#include "hw/9pfs/9p-util.h" > > #include "fsdev/9p-iov-marshal.h" > > > > #define PROGNAME "virtfs-proxy-helper" > > @@ -338,6 +339,49 @@ static void resetugid(int suid, int sgid) > > } > > } > > > > +/* > > + * Open regular file or directory. Attempts to open any special file are > > + * rejected. > > + * > > + * returns file descriptor or -1 on error > > + */ > > +static int open_regular(const char *pathname, int flags, mode_t mode) { > > + int fd; > > + struct stat stbuf; > > + > > + fd = open(pathname, flags, mode); > > + if (fd < 0) { > > + return fd; > > + } > > + > > + /* CVE-2023-2861: Prohibit opening any special file directly on host > > + * (especially device files), as a compromised client could potentially > > + * gain access outside exported tree under certain, unsafe setups. We > > + * expect client to handle I/O on special files exclusively on guest > > side. > > + */ > > + if (qemu_fstat(fd, &stbuf) < 0) { > > + close_preserve_errno(fd); > > + return -1; > > + } > > + if (!S_ISREG(stbuf.st_mode) && !S_ISDIR(stbuf.st_mode)) { > > + /* Tcreate and Tlcreate 9p messages mandate to immediately open the > > + * created file for I/O. So this is not (necessarily) due to a > > broken > > + * client, and hence no error message is to be reported in this > > case. > > + */ > > + if (!(flags & O_CREAT)) { > > Tlcreate is explicitly about creating regular files only (see [1] and > v9fs_lcreate()) and I don't quite see how open() could successfully > create a regular file and the resulting fd is fstat'ed as something > else. > > Tcreate seems to cover more types but again only regular files (with O_CREAT) > or directories (without O_CREAT) are expected here (see v9fs_create()). > > Unless I'm missing something, it seems that the comment and the O_CREAT > check should be removed. > > [1] > https://github.com/chaos/diod/blob/master/protocol.md#lcreate----create-regular-file You are right about Tlcreate, but for Tcreate 9p2000.u specifies, quote: "In addition to creating directories with DMDIR, 9P2000.u allows the creation of symlinks (DMSYMLINK), devices (DMDEVICE), named pipes (DMNAMEPIPE), and sockets (DMSOCKET)." http://ericvh.github.io/9p-rfc/rfc9p2000.u.html#anchor17 So I just remove mentioning Tlcreate in the comment? > > > + error_report_once( > > + "9p: broken or compromised client detected; attempt to > > open " > > + "special file (i.e. neither regular file, nor directory)" > > + ); > > + } > > + close(fd); > > + errno = ENXIO; > > + return -1; > > + } > > + > > + return fd; > > +} > > + > > /* > > * send response in two parts > > * 1) ProxyHeader > > @@ -682,7 +726,7 @@ static int do_create(struct iovec *iovec) > > if (ret < 0) { > > goto unmarshal_err_out; > > } > > - ret = open(path.data, flags, mode); > > + ret = open_regular(path.data, flags, mode); > > if (ret < 0) { > > ret = -errno; > > } > > @@ -707,7 +751,7 @@ static int do_open(struct iovec *iovec) > > if (ret < 0) { > > goto err_out; > > } > > - ret = open(path.data, flags); > > + ret = open_regular(path.data, flags, 0); > > if (ret < 0) { > > ret = -errno; > > } > > diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h > > index c314cf381d..9da1a0538d 100644 > > --- a/hw/9pfs/9p-util.h > > +++ b/hw/9pfs/9p-util.h > > @@ -13,6 +13,8 @@ > > #ifndef QEMU_9P_UTIL_H > > #define QEMU_9P_UTIL_H > > > > +#include "qemu/error-report.h" > > + > > #ifdef O_PATH > > #define O_PATH_9P_UTIL O_PATH > > #else > > @@ -95,6 +97,7 @@ static inline int errno_to_dotl(int err) { > > #endif > > > > #define qemu_openat openat > > +#define qemu_fstat fstat > > #define qemu_fstatat fstatat > > #define qemu_mkdirat mkdirat > > #define qemu_renameat renameat > > @@ -118,6 +121,7 @@ static inline int openat_file(int dirfd, const char > > *name, int flags, > > mode_t mode) > > { > > int fd, serrno, ret; > > + struct stat stbuf; > > > > #ifndef CONFIG_DARWIN > > again: > > @@ -142,6 +146,31 @@ again: > > return -1; > > } > > > > + /* CVE-2023-2861: Prohibit opening any special file directly on host > > + * (especially device files), as a compromised client could potentially > > + * gain access outside exported tree under certain, unsafe setups. We > > + * expect client to handle I/O on special files exclusively on guest > > side. > > + */ > > + if (qemu_fstat(fd, &stbuf) < 0) { > > + close_preserve_errno(fd); > > + return -1; > > + } > > + if (!S_ISREG(stbuf.st_mode) && !S_ISDIR(stbuf.st_mode)) { > > + /* Tcreate and Tlcreate 9p messages mandate to immediately open the > > + * created file for I/O. So this is not (necessarily) due to a > > broken > > + * client, and hence no error message is to be reported in this > > case. > > + */ > > Same remark as with the proxy helper. > > If you agree with my suggestions, feel free to add my R-b right away. > > Cheers, I'll definitely take the time for another (v3) round in this case. Thanks! > -- > Greg > > > + if (!(flags & O_CREAT)) { > > + error_report_once( > > + "9p: broken or compromised client detected; attempt to > > open " > > + "special file (i.e. neither regular file, nor directory)" > > + ); > > + } > > + close(fd); > > + errno = ENXIO; > > + return -1; > > + } > > + > > serrno = errno; > > /* O_NONBLOCK was only needed to open the file. Let's drop it. We don't > > * do that with O_PATH since fcntl(F_SETFL) isn't supported, and > > openat() > >