On Freitag, 28. Januar 2022 23:33:26 CET Vitaly Chikunov wrote: > `struct dirent' returned from readdir(3) could be shorter than > `sizeof(struct dirent)', thus memcpy of sizeof length will overread > into unallocated page causing SIGSEGV. Example stack trace:
I actually suggested to make it clear in the commit log comment here that it could be both, shorter or longer, but OK, the API doc comment on qemu_dirent_dup() sais it, so ... Reviewed-by: Christian Schoenebeck <[email protected]> > > #0 0x00005555559ebeed v9fs_co_readdir_many (/usr/bin/qemu-system-x86_64 + > 0x497eed) #1 0x00005555559ec2e9 v9fs_readdir (/usr/bin/qemu-system-x86_64 > + 0x4982e9) #2 0x0000555555eb7983 coroutine_trampoline > (/usr/bin/qemu-system-x86_64 + 0x963983) #3 0x00007ffff73e0be0 n/a (n/a + > 0x0) > > While fixing, provide a helper for any future `struct dirent' cloning. > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/841 > Cc: [email protected] > Co-authored-by: Christian Schoenebeck <[email protected]> > Signed-off-by: Vitaly Chikunov <[email protected]> > --- > Tested on x86-64 Linux. > > Changes since v1: > - Update commentary text. > - Remove hanging of g_malloc "errors". > - Simplify qemu_dirent_dup. > > hw/9pfs/codir.c | 3 +-- > include/qemu/osdep.h | 13 +++++++++++++ > util/osdep.c | 16 ++++++++++++++++ > 3 files changed, 30 insertions(+), 2 deletions(-) > > diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c > index 032cce04c4..c0873bde16 100644 > --- a/hw/9pfs/codir.c > +++ b/hw/9pfs/codir.c > @@ -143,8 +143,7 @@ static int do_readdir_many(V9fsPDU *pdu, V9fsFidState > *fidp, } else { > e = e->next = g_malloc0(sizeof(V9fsDirEnt)); > } > - e->dent = g_malloc0(sizeof(struct dirent)); > - memcpy(e->dent, dent, sizeof(struct dirent)); > + e->dent = qemu_dirent_dup(dent); > > /* perform a full stat() for directory entry if requested by caller > */ if (dostat) { > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h > index d1660d67fa..ce12f64853 100644 > --- a/include/qemu/osdep.h > +++ b/include/qemu/osdep.h > @@ -805,6 +805,19 @@ static inline int > platform_does_not_support_system(const char *command) } > #endif /* !HAVE_SYSTEM_FUNCTION */ > > +/** > + * Duplicate directory entry @dent. > + * > + * It is highly recommended to use this function instead of open coding > + * duplication of @c dirent objects, because the actual @c struct @c dirent > + * size may be bigger or shorter than @c sizeof(struct dirent) and correct > + * handling is platform specific (see gitlab issue #841). > + * > + * @dent - original directory entry to be duplicated > + * @returns duplicated directory entry which should be freed with g_free() > + */ > +struct dirent *qemu_dirent_dup(struct dirent *dent); > + > #ifdef __cplusplus > } > #endif > diff --git a/util/osdep.c b/util/osdep.c > index 42a0a4986a..0bc7ec1e22 100644 > --- a/util/osdep.c > +++ b/util/osdep.c > @@ -33,6 +33,7 @@ > extern int madvise(char *, size_t, int); > #endif > > +#include <dirent.h> > #include "qemu-common.h" > #include "qemu/cutils.h" > #include "qemu/sockets.h" > @@ -615,3 +616,18 @@ writev(int fd, const struct iovec *iov, int iov_cnt) > return readv_writev(fd, iov, iov_cnt, true); > } > #endif > + > +struct dirent * > +qemu_dirent_dup(struct dirent *dent) > +{ > +#if defined _DIRENT_HAVE_D_RECLEN > + /* Avoid use of strlen() if there's d_reclen. */ > + const size_t sz = dent->d_reclen; > +#else > + /* Fallback to a most portable way. */ > + const size_t sz = offsetof(struct dirent, d_name) + > + strlen(dent->d_name) + 1; > +#endif > + struct dirent *dst = g_malloc(sz); > + return memcpy(dst, dent, sz); > +}
