Re: [Qemu-devel] [PATCH] 9pfs: fix vulnerability in openat_dir() and local_unlinkat_common()

[Mark Cave-Ayland]

On 03/03/17 17:25, Greg Kurz wrote:

> We should pass O_NOFOLLOW otherwise openat() will follow symlinks and make
> QEMU vulnerable.
> 
> O_PATH was used as an optimization: the fd returned by openat_dir() is only
> passed to openat() actually, so we don't really need to reach the underlying
> filesystem.
> 
> O_NOFOLLOW | O_PATH isn't an option: if name is a symlink, openat() will
> return a fd, forcing us to do some other syscall to detect we have a
> symlink. Also, O_PATH doesn't exist in glibc 2.13 and older.
> 
> The only sane thing to do is hence to drop O_PATH, and only pass O_NOFOLLOW.
> 
> While here, we also fix local_unlinkat_common() to use openat_dir() for
> the same reasons (it was a leftover in the original patchset actually).
> 
> This fixes CVE-2016-9602.
> 
> Signed-off-by: Greg Kurz <address@hidden>
> ---
>  hw/9pfs/9p-local.c |    2 +-
>  hw/9pfs/9p-util.h  |    2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
> index 5db7104334d6..e31309a29c58 100644
> --- a/hw/9pfs/9p-local.c
> +++ b/hw/9pfs/9p-local.c
> @@ -959,7 +959,7 @@ static int local_unlinkat_common(FsContext *ctx, int 
> dirfd, const char *name,
>          if (flags == AT_REMOVEDIR) {
>              int fd;
>  
> -            fd = openat(dirfd, name, O_RDONLY | O_DIRECTORY | O_PATH);
> +            fd = openat_dir(dirfd, name);
>              if (fd == -1) {
>                  goto err_out;
>              }
> diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
> index 091f3ce88e15..4001d1fd8398 100644
> --- a/hw/9pfs/9p-util.h
> +++ b/hw/9pfs/9p-util.h
> @@ -22,7 +22,7 @@ static inline void close_preserve_errno(int fd)
>  
>  static inline int openat_dir(int dirfd, const char *name)
>  {
> -    return openat(dirfd, name, O_DIRECTORY | O_RDONLY | O_PATH);
> +    return openat(dirfd, name, O_DIRECTORY | O_RDONLY | O_NOFOLLOW);
>  }
>  
>  static inline int openat_file(int dirfd, const char *name, int flags,

Thanks Greg.

This patch got me slightly further, but I now get another build failure:


cc -I/home/build/src/qemu/git/qemu/hw/9pfs -Ihw/9pfs
-I/home/build/src/qemu/git/qemu/tcg
-I/home/build/src/qemu/git/qemu/tcg/i386
-I/home/build/src/qemu/git/qemu/linux-headers
-I/home/build/src/qemu/git/qemu/linux-headers -I.
-I/home/build/src/qemu/git/qemu -I/home/build/src/qemu/git/qemu/include
-I/usr/include/pixman-1   -I/home/build/src/qemu/git/qemu/dtc/libfdt
-Werror -pthread -I/usr/include/glib-2.0
-I/usr/lib/x86_64-linux-gnu/glib-2.0/include   -m64 -mcx16 -D_GNU_SOURCE
-D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -Wstrict-prototypes
-Wredundant-decls -Wall -Wundef -Wwrite-strings -Wmissing-prototypes
-fno-strict-aliasing -fno-common -fwrapv  -Wendif-labels
-Wno-missing-include-dirs -Wempty-body -Wnested-externs
-Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers
-Wold-style-declaration -Wold-style-definition -Wtype-limits
-fstack-protector-all -I/usr/include/p11-kit-1
-I/usr/include/libpng12   -I/home/build/src/qemu/git/qemu/tests -MMD -MP
-MT hw/9pfs/9p-local.o -MF hw/9pfs/9p-local.d -O2 -U_FORTIFY_SOURCE
-D_FORTIFY_SOURCE=2 -g   -c -o hw/9pfs/9p-local.o hw/9pfs/9p-local.c
hw/9pfs/9p-local.c: In function ‘local_set_cred_passthrough’:
hw/9pfs/9p-local.c:352:40: error: ‘AT_EMPTY_PATH’ undeclared (first use
in this function)
hw/9pfs/9p-local.c:352:40: note: each undeclared identifier is reported
only once for each function it appears in
make: *** [hw/9pfs/9p-local.o] Error 1


A quick search suggested that I should be able to get around this by
adding a #include in hw/9pfs/9p-local.c for "linux/fcntl.h" however
adding that gives me a couple of redefinition errors:


cc -I/home/build/src/qemu/git/qemu/hw/9pfs -Ihw/9pfs
-I/home/build/src/qemu/git/qemu/tcg
-I/home/build/src/qemu/git/qemu/tcg/i386
-I/home/build/src/qemu/git/qemu/linux-headers
-I/home/build/src/qemu/git/qemu/linux-headers -I.
-I/home/build/src/qemu/git/qemu -I/home/build/src/qemu/git/qemu/include
-I/usr/include/pixman-1   -I/home/build/src/qemu/git/qemu/dtc/libfdt
-Werror -pthread -I/usr/include/glib-2.0
-I/usr/lib/x86_64-linux-gnu/glib-2.0/include   -m64 -mcx16 -D_GNU_SOURCE
-D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -Wstrict-prototypes
-Wredundant-decls -Wall -Wundef -Wwrite-strings -Wmissing-prototypes
-fno-strict-aliasing -fno-common -fwrapv  -Wendif-labels
-Wno-missing-include-dirs -Wempty-body -Wnested-externs
-Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers
-Wold-style-declaration -Wold-style-definition -Wtype-limits
-fstack-protector-all -I/usr/include/p11-kit-1
-I/usr/include/libpng12   -I/home/build/src/qemu/git/qemu/tests -MMD -MP
-MT hw/9pfs/9p-local.o -MF hw/9pfs/9p-local.d -O2 -U_FORTIFY_SOURCE
-D_FORTIFY_SOURCE=2 -g   -c -o hw/9pfs/9p-local.o hw/9pfs/9p-local.c
In file included from /usr/include/x86_64-linux-gnu/asm/fcntl.h:1:0,
                 from /usr/include/linux/fcntl.h:4,
                 from hw/9pfs/9p-local.c:30:
/usr/include/asm-generic/fcntl.h:127:8: error: redefinition of ‘struct
f_owner_ex’
In file included from /usr/include/fcntl.h:34:0,
                 from /home/build/src/qemu/git/qemu/include/qemu/osdep.h:79,
                 from hw/9pfs/9p-local.c:14:
/usr/include/x86_64-linux-gnu/bits/fcntl.h:203:8: note: originally
defined here
In file included from /usr/include/x86_64-linux-gnu/asm/fcntl.h:1:0,
                 from /usr/include/linux/fcntl.h:4,
                 from hw/9pfs/9p-local.c:30:
/usr/include/asm-generic/fcntl.h:167:8: error: redefinition of ‘struct
flock’
In file included from /usr/include/fcntl.h:34:0,
                 from /home/build/src/qemu/git/qemu/include/qemu/osdep.h:79,
                 from hw/9pfs/9p-local.c:14:
/usr/include/x86_64-linux-gnu/bits/fcntl.h:167:8: note: originally
defined here
In file included from /usr/include/x86_64-linux-gnu/asm/fcntl.h:1:0,
                 from /usr/include/linux/fcntl.h:4,
                 from hw/9pfs/9p-local.c:30:
/usr/include/asm-generic/fcntl.h:184:8: error: redefinition of ‘struct
flock64’
In file included from /usr/include/fcntl.h:34:0,
                 from /home/build/src/qemu/git/qemu/include/qemu/osdep.h:79,
                 from hw/9pfs/9p-local.c:14:
/usr/include/x86_64-linux-gnu/bits/fcntl.h:182:8: note: originally
defined here
make: *** [hw/9pfs/9p-local.o] Error 1


Any thoughts?

Mark.