linux-kernel - Re: [PATCH][next] scsi: lpfc: Avoid -Wstringop-overflow warning

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Message-ID: <202306010931.92796DC@keescook>
Date:   Thu, 1 Jun 2023 09:48:55 -0700
From:   Kees Cook <keescook@...omium.org>
To:     James Bottomley <jejb@...ux.ibm.com>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        James Smart <james.smart@...adcom.com>,
        Dick Kennedy <dick.kennedy@...adcom.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] scsi: lpfc: Avoid -Wstringop-overflow warning

On Wed, May 31, 2023 at 10:56:50AM -0400, James Bottomley wrote:
> On Tue, 2023-05-30 at 15:44 -0700, Kees Cook wrote:
> > On Tue, May 30, 2023 at 05:36:06PM -0400, James Bottomley wrote:
> > > On Tue, 2023-05-30 at 15:30 -0600, Gustavo A. R. Silva wrote:
> > > > Avoid confusing the compiler about possible negative sizes.
> > > > Use size_t instead of int for variables size and copied.
> > > > 
> > > > Address the following warning found with GCC-13:
> > > > In function ‘lpfc_debugfs_ras_log_data’,
> > > >     inlined from ‘lpfc_debugfs_ras_log_open’ at
> > > > drivers/scsi/lpfc/lpfc_debugfs.c:2271:15:
> > > > drivers/scsi/lpfc/lpfc_debugfs.c:2210:25: warning: ‘memcpy’
> > > > specified
> > > > bound between 18446744071562067968 and 18446744073709551615
> > > > exceeds
> > > > maximum object size 9223372036854775807 [-Wstringop-overflow=]
> > > >  2210 |                         memcpy(buffer + copied, dmabuf-
> > > > >virt,
> > > >       |                        
> > > > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > >  2211 |                                size - copied - 1);
> > > >       |                                ~~~~~~~~~~~~~~~~~~
> > > > 
> > > 
> > > This looks like a compiler bug to me and your workaround would have
> > > us using unsigned types everywhere for sizes, which seems wrong. 
> > > There are calls which return size or error for which we have
> > > ssize_t and that type has to be usable in things like memcpy, so
> > > the compiler must be fixed or the warning disabled.
> > 
> > The compiler is (correctly) noticing that the calculation involving
> > "size" (from which "copied" is set) could go negative.
> 
> It can?  But if it can, then changing size and copied to unsigned
> doesn't fix it, does it?

Yes:

	(int)	(const expression 256 * 1024)		(u32)
        size = LPFC_RAS_MIN_BUFF_POST_SIZE * phba->cfg_ras_fwlog_buffsize;

this can wrap to negative if cfg_ras_fwlog_buffsize is large enough. If
"size" is size_t, it can't wrap, and is therefore never negative.

> So your claim is the compiler only gets it wrong in this one case and
> if we just change this one case it will never get it wrong again?

What? No, I'm saying this is a legitimate diagnostic, and the wrong type
was chosen for "size": it never needs to carry a negative value, and it
potentially needs to handle values greater than u32.

But you're right -- there is still a potential for runtime confusion in
that the return from lpfc_debugfs_ras_log_data() must be signed. So
perhaps the best option is to check for overflow directly.

Gustavo, does this fix it?


diff --git a/drivers/scsi/lpfc/lpfc_debugfs.c b/drivers/scsi/lpfc/lpfc_debugfs.c
index bdf34af4ef36..7f9b221e7c34 100644
--- a/drivers/scsi/lpfc/lpfc_debugfs.c
+++ b/drivers/scsi/lpfc/lpfc_debugfs.c
@@ -2259,11 +2259,15 @@ lpfc_debugfs_ras_log_open(struct inode *inode, struct file *file)
 		goto out;
 	}
 	spin_unlock_irq(&phba->hbalock);
-	debug = kmalloc(sizeof(*debug), GFP_KERNEL);
+
+	if (check_mul_overflow(LPFC_RAS_MIN_BUFF_POST_SIZE,
+			       phba->cfg_ras_fwlog_buffsize, &size))
+		goto out;
+
+	debug = kzalloc(sizeof(*debug), GFP_KERNEL);
 	if (!debug)
 		goto out;
 
-	size = LPFC_RAS_MIN_BUFF_POST_SIZE * phba->cfg_ras_fwlog_buffsize;
 	debug->buffer = vmalloc(size);
 	if (!debug->buffer)
 		goto free_debug;


-Kees

-- 
Kees Cook