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    SubjectRe: [PATCH 6/8] ASoC: intel: skylake: change snprintf to scnprintf for possible overflow
    On Sat, Jan 12, 2019 at 7:28 AM Willy Tarreau <w@1wt.eu> wrote:
    >
    > From: Silvio Cesare <silvio.cesare@gmail.com>
    >
    > Change snprintf to scnprintf. There are generally two cases where using
    > snprintf causes problems.
    >
    > 1) Uses of size += snprintf(buf, SIZE - size, fmt, ...)
    > In this case, if snprintf would have written more characters than what the
    > buffer size (SIZE) is, then size will end up larger than SIZE. In later
    > uses of snprintf, SIZE - size will result in a negative number, leading
    > to problems. Note that size might already be too large by using
    > size = snprintf before the code reaches a case of size += snprintf.
    >
    > 2) If size is ultimately used as a length parameter for a copy back to user
    > space, then it will potentially allow for a buffer overflow and information
    > disclosure when size is greater than SIZE. When the size is used to index
    > the buffer directly, we can have memory corruption. This also means when
    > size = snprintf... is used, it may also cause problems since size may become
    > large. Copying to userspace is mitigated by the HARDENED_USERCOPY kernel
    > configuration.
    >
    > The solution to these issues is to use scnprintf which returns the number of
    > characters actually written to the buffer, so the size variable will never
    > exceed SIZE.
    >
    > Signed-off-by: Silvio Cesare <silvio.cesare@gmail.com>
    > Cc: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
    > Cc: Liam Girdwood <liam.r.girdwood@linux.intel.com>
    > Cc: Jie Yang <yang.jie@linux.intel.com>
    > Cc: Dan Carpenter <dan.carpenter@oracle.com>
    > Cc: Kees Cook <keescook@chromium.org>
    > Cc: Will Deacon <will.deacon@arm.com>
    > Cc: Greg KH <greg@kroah.com>
    > Signed-off-by: Willy Tarreau <w@1wt.eu>
    >
    > ---
    > sound/soc/intel/skylake/skl-debug.c | 28 ++++++++++++++--------------
    > 1 file changed, 14 insertions(+), 14 deletions(-)
    >
    > diff --git a/sound/soc/intel/skylake/skl-debug.c b/sound/soc/intel/skylake/skl-debug.c
    > index 5d7ac2ee7a3c..bb28db734fb7 100644
    > --- a/sound/soc/intel/skylake/skl-debug.c
    > +++ b/sound/soc/intel/skylake/skl-debug.c
    > @@ -43,7 +43,7 @@ static ssize_t skl_print_pins(struct skl_module_pin *m_pin, char *buf,
    > ssize_t ret = 0;
    >
    > for (i = 0; i < max_pin; i++)
    > - ret += snprintf(buf + size, MOD_BUF - size,
    > + ret += scnprintf(buf + size, MOD_BUF - size,
    > "%s %d\n\tModule %d\n\tInstance %d\n\t"
    > "In-used %s\n\tType %s\n"
    > "\tState %d\n\tIndex %d\n",
    >

    While working on a Coccinelle script to find more cases of this, I
    noticed that this code is buggy: it keeps overwriting the same
    position in the buf string: "buf + size" and don't take "ret" into
    account at all. This needs to be:

    ret += scnprintf(buf + size + ret, MOD_BUF - size - ret,

    --
    Kees Cook

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