Messages in this thread | | | Date | Tue, 13 May 2014 19:34:33 +0200 | From | "Michael Kerrisk (man-pages)" <> | Subject | Re: [PATCH 1/1 v2] fanotify: check file flags passed in fanotify_init |
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On 05/13/2014 07:16 PM, Heinrich Schuchardt wrote: > On 13.05.2014 10:25, Michael Kerrisk (man-pages) wrote: >> Hello Heinrich >> >> On Mon, May 12, 2014 at 9:02 PM, Heinrich Schuchardt <xypron.glpk@gmx.de> wrote: >>> Without this patch fanotify_init does not validate the value passed in >>> event_f_flags. >>> >>> When a fanotify event is read from the fanotify file descriptor a new file >>> descriptor is created where file.f_flags = event_f_flags. >>> >>> Internal and external open flags are stored together in field f_flags of >>> struct file. Hence, an application might create file descriptors with >>> internal flags like FMODE_EXEC, FMODE_NOCMTIME set. >>> >>> Jan Kara and Eric Paris both aggreed that this is a bug and the value of >>> event_f_flags should be checked: >>> https://lkml.org/lkml/2014/4/29/522 >>> https://lkml.org/lkml/2014/4/29/539 >>> >>> This updated patch version considers the comments by Michael Kerrisk in >>> https://lkml.org/lkml/2014/5/4/10 >>> >>> With the patch the value of event_f_flags is checked. >>> When specifying an invalid value error EINVAL is returned. >>> >>> Internal flags are disallowed. >>> >>> File creation flags are disallowed: >>> O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TRUNC, and O_TTY_INIT. >>> >>> Flags which do not make sense with fanotify are disallowed: >>> __O_TMPFILE, O_PATH, FASYNC, and O_DIRECT. >>> >>> This leaves us with the following allowed values: >>> >>> O_RDONLY, O_WRONLY, O_RDWR are basic functionality. The are stored in the >>> bits given by O_ACCMODE. >>> >>> O_APPEND is working as expected. The value might be useful in a logging >>> application which appends the current status each time the log is opened. >>> >>> O_LARGEFILE is needed for files exceeding 4GB on 32bit systems. >>> >>> O_NONBLOCK may be useful when monitoring slow devices like tapes. >>> >>> O_NDELAY is equal to O_NONBLOCK except for platform parisc. >>> To avoid code breaking on parisc either both flags should be >>> allowed or none. The patch allows both. >>> >>> __O_SYNC and O_DSYNC may be used to avoid data loss on power disruption. >>> >>> O_NOATIME may be useful to reduce disk activity. >>> >>> O_CLOEXEC may be useful, if separate processes shall be used to scan files. >>> >>> Once this patch is accepted, the fanotify_init.2 manpage has to be updated. >>> >>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de> >>> --- >>> fs/notify/fanotify/fanotify_user.c | 25 +++++++++++++++++++++++++ >>> 1 file changed, 25 insertions(+) >>> >>> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c >>> index d42220f..31b0de2 100644 >>> --- a/fs/notify/fanotify/fanotify_user.c >>> +++ b/fs/notify/fanotify/fanotify_user.c >>> @@ -25,6 +25,19 @@ >>> #define FANOTIFY_DEFAULT_MAX_MARKS 8192 >>> #define FANOTIFY_DEFAULT_MAX_LISTENERS 128 >>> >>> +/* >>> + * All flags that may be specified in parameter event_f_flags of fanotify_init. >>> + * >>> + * Internal and external open flags are stored together in field f_flags of >>> + * struct file. Only external open flags shall be allowed in event_f_flags. >>> + * Internal flags like FMODE_NONOTIFY, FMODE_EXEC, FMODE_NOCMTIME shall be >>> + * excluded. >>> + */ >>> +#define FANOTIFY_INIT_ALL_EVENT_F_BITS ( \ >>> + O_ACCMODE | O_APPEND | O_NONBLOCK | \ >>> + __O_SYNC | O_DSYNC | O_CLOEXEC | \ >>> + O_LARGEFILE | O_NOATIME ) >>> + >>> extern const struct fsnotify_ops fanotify_fsnotify_ops; >> >> The above looks okay to me, but I'd be happier seeing an Ack from Jan or Eric. >> >>> static struct kmem_cache *fanotify_mark_cache __read_mostly; >>> @@ -669,6 +682,18 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags) >>> if (flags & ~FAN_ALL_INIT_FLAGS) >>> return -EINVAL; >>> >>> + if (event_f_flags & ~FANOTIFY_INIT_ALL_EVENT_F_BITS) >>> + return -EINVAL; >>> + >>> + switch (event_f_flags & O_ACCMODE) { >>> + case O_RDONLY: >>> + case O_RDWR: >>> + case O_WRONLY: >>> + break; >>> + default: >>> + return -EINVAL; >>> + } >>> + >> >> The 'switch' could just be replaced by: >> >> if ((event_f_flags & O_ACCMODE) == 3) >> return -EINVAL; >> >> (But I'm not sure if some might prefer the idiom you have used.) >> >> Cheers, >> >> Michael > > Using explicit numbers makes the code hard to read and analyze. > I do not intend to participate in the Obfuscated C Code Contest.
I agree that using explicit numbers is odd, but '3" in this context does have a special meaning (see open(2)), and it struck me as a more efficient way of doing the check. But, if others are fine with what you've written, I have no real problem with it.
Cheers,
Michael
-- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
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