[PATCH] mtdchar: fix overflows in adjustment of `count`

[boris.brezillon@xxxxxxxxxxx (Boris Brezillon)]

On Sat, 7 Jul 2018 11:03:00 +0200
Jann Horn <jannh at google.com> wrote:

> +cc linux-api
> 
> On Sat, Jul 7, 2018 at 10:44 AM Boris Brezillon
> <boris.brezillon at bootlin.com> wrote:
> >
> > On Sat,  7 Jul 2018 05:37:22 +0200
> > Jann Horn <jannh at google.com> wrote:
> >  
> > > The first checks in mtdchar_read() and mtdchar_write() attempt to limit
> > > `count` such that `*ppos + count <= mtd->size`. However, they ignore the
> > > possibility of `*ppos > mtd->size`, allowing the calculation of `count` to
> > > wrap around. `mtdchar_lseek()` prevents seeking beyond mtd->size, but the
> > > pread/pwrite syscalls bypass this.
> > >
> > > I haven't found any codepath on which this actually causes dangerous
> > > behavior, but it seems like a sensible change anyway.
> > >
> > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > > Signed-off-by: Jann Horn <jannh at google.com>
> > > ---
> > >  drivers/mtd/mtdchar.c | 10 +++++++---
> > >  1 file changed, 7 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c
> > > index cd67c85cc87d..02389528f622 100644
> > > --- a/drivers/mtd/mtdchar.c
> > > +++ b/drivers/mtd/mtdchar.c
> > > @@ -160,8 +160,12 @@ static ssize_t mtdchar_read(struct file *file, char __user *buf, size_t count,
> > >
> > >       pr_debug("MTD_read\n");
> > >
> > > -     if (*ppos + count > mtd->size)
> > > -             count = mtd->size - *ppos;
> > > +     if (*ppos + count > mtd->size) {
> > > +             if (*ppos < mtd->size)
> > > +                     count = mtd->size - *ppos;
> > > +             else
> > > +                     count = 0;
> > > +     }  
> >
> > Hm, shouldn't we return -ERANGE or -EINVAL if *ppos >= mtd->size?  
> 
> Hmm, good question.
> The pread() manpage says that pread() can return the same errors as
> lseek(), and the lseek() manpage says that -EINVAL is for when you're
> trying to seek beyond the end of a seekable device. So from the
> documentation, it sounds as if you're right.
> But testing pread() beyond end of file for various things on my
> machine seems to just return 0:
> 
> # cat pread.c
> #include <unistd.h>
> #include <stdlib.h>
> int main(int argc, char **argv) {
>   char buf[0x1000];
>   off_t off = strtoul(argv[1], NULL, 0);
>   pread(0, buf, 0x1000, off);
> }
> # gcc -o pread pread.c
> # strace -e trace=pread64 ./pread 200000000 < /dev/sda1
> pread64(0, "", 4096, 200000000)         = 0
> +++ exited with 0 +++
> # strace -e trace=pread64 ./pread 100000 <
> /sys/kernel/debug/x86/tlb_single_page_flush_ceiling
> pread64(0, "", 4096, 100000)            = 0
> +++ exited with 0 +++
> # strace -e trace=pread64 ./pread 20000000000 < /dev/dm-2
> pread64(0, "", 4096, 20000000000)       = 0
> +++ exited with 0 +++
> 
> Do you know of precedent for returning -EINVAL if *ppos is beyond the
> end of the device?

Nope, it just made more sense to me than returning 0. Anyway, let's
keep the most common behavior, even if it's not documented this way ;-).

> 
> > >
> > >       if (!count)
> > >               return 0;
> > > @@ -246,7 +250,7 @@ static ssize_t mtdchar_write(struct file *file, const char __user *buf, size_t c
> > >
> > >       pr_debug("MTD_write\n");
> > >
> > > -     if (*ppos == mtd->size)
> > > +     if (*ppos >= mtd->size)
> > >               return -ENOSPC;
> > >
> > >       if (*ppos + count > mtd->size)  
> >  
> 
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