Re: [PATCH v2 next] ACPI: APEI: EINJ: prevent memory corruption in error_type_set()

[Dan Carpenter]

On Wed, Jun 25, 2025 at 08:04:14PM +0200, Uwe Kleine-König wrote:
> Hello Dan,
> 
> On Wed, Jun 25, 2025 at 11:34:38AM -0500, Dan Carpenter wrote:
> > The "einj_buf" buffer is 32 chars.  Verify that "count" is not too large
> > for that.  Also leave the last character as a NUL terminator to ensure
> > the string is properly terminated.
> > 
> > Fixes: 0c6176e1e186 ("ACPI: APEI: EINJ: Enable the discovery of EINJv2 capabilities")
> > Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > ---
> > v2: I messed up the sizeof() calculation in the copy_from_user() and I put
> >     the parentheses in the wrong place in v1.
> > 
> >  drivers/acpi/apei/einj-core.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/acpi/apei/einj-core.c b/drivers/acpi/apei/einj-core.c
> > index d6d7e36e3647..2206cbbdccfa 100644
> > --- a/drivers/acpi/apei/einj-core.c
> > +++ b/drivers/acpi/apei/einj-core.c
> > @@ -826,8 +826,11 @@ static ssize_t error_type_set(struct file *file, const char __user *buf,
> >  	int rc;
> >  	u64 val;
> >  
> > +	if (count > sizeof(einj_buf))
> > +		return -EINVAL;
> > +
> >  	memset(einj_buf, 0, sizeof(einj_buf));
> > -	if (copy_from_user(einj_buf, buf, count))
> 
> I would add a comment here to explain the -1. It's in the commit log,
> but that doesn't help a future reader.
> 

Sure.  I can add a comment.

	/* Leave the last character for the NUL terminator */

> > +	if (copy_from_user(einj_buf, buf, min(count, sizeof(einj_buf) - 1)))
> >  		return -EFAULT;
> >  
> >  	if (strncmp(einj_buf, "V2_", 3) == 0) {
> 
> Further down there is:
> 
> 	return count;
> 
> Is that still correct given that you read less now?

Yep.  If we don't return count userspace will try again, right?

regards,
dan carpenter