Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread

["Philippe Mathieu-Daudé via" <qemu-devel@nongnu.org>]

On 5/2/22 12:36, Christian Schoenebeck wrote:
> On Freitag, 4. Februar 2022 06:06:09 CET Vitaly Chikunov wrote:
>> `struct dirent' returned from readdir(3) could be shorter (or longer)
>> than `sizeof(struct dirent)', thus memcpy of sizeof length will overread
>> into unallocated page causing SIGSEGV. Example stack trace:
>>
>>   #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: qemu-stable@nongnu.org
>> Co-authored-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
>> Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
>> ---
>> Tested on x86-64 Linux again.
>>
>> Changes from v2:
>> - Make it work with a simulated dirent where d_reclen is 0, which was
>>    caused abort in readdir qos-test, by using fallback at runtime.
>>
>>   hw/9pfs/codir.c      |  3 +--
>>   include/qemu/osdep.h | 13 +++++++++++++
>>   util/osdep.c         | 18 ++++++++++++++++++
>>   3 files changed, 32 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..2c80528a61 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 +616,20 @@ 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)
>> +{
>> +    size_t sz = 0;
>> +#if defined _DIRENT_HAVE_D_RECLEN
>> +    /* Avoid use of strlen() if there's d_reclen. */
>> +    sz = dent->d_reclen;
>> +#endif
>> +    if (sz == 0) {
> 
> Philippe, Greg, apart from the additional comment, do you want to see this
> check to be changed in v4 to this?
> 
> 	if (unlikely(sz == 0)) {

This is not an hot path so probably not very useful.

> 
> Best regards,
> Christian Schoenebeck
> 
>> +        /* Fallback to the most portable way. */
>> +        sz = offsetof(struct dirent, d_name) +
>> +                      strlen(dent->d_name) + 1;
>> +    }
>> +    struct dirent *dst = g_malloc(sz);
>> +    return memcpy(dst, dent, sz);

However 'return g_memdup(dent, sz)' is simpler to review.

>> +}
> 
> 
>