linux-kernel - Re: [PATCH][V2][Fix commit message] x86/RAS/mce_amd_inj: fix signed wrap around when decrementing index i

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Date:   Sat, 17 Sep 2016 09:35:11 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Colin King <colin.king@...onical.com>
Cc:     Yazen Ghannam <Yazen.Ghannam@....com>,
        linux-kernel@...r.kernel.org,
        Aravind Gopalakrishnan <aravindksg.lkml@...il.com>
Subject: Re: [PATCH][V2][Fix commit message] x86/RAS/mce_amd_inj: fix signed
 wrap around when decrementing index i

(drop peterz from CC).

On Fri, Sep 16, 2016 at 12:22:23PM +0100, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
> 
> Integer index i needs to be a signed int rather than unsigned to avoid
> a wrap-around when decrementing in the while loop.  For example, if the

Damn corner cases :-\

> debugfs_create_file fails when i is zero, the current situation will
> predecrement i in the while loop, wrapping i to the maximum signed
> integer and cause multiple out of bounds reads on dfs_fls[i].d as
> the loop interates to zero.
> 
> Also add (int) cast to fix warning that the original fix attempted
> to fix.

Thanks, that's a good catch!

But, can we keep the unsigned-ness of i as it is an index and do this
instead?

(Oh, and make it return ENODEV - that ENOMEM is just plain wrong most of
the time).

---
diff --git a/arch/x86/ras/mce_amd_inj.c b/arch/x86/ras/mce_amd_inj.c
index cd318d93099e..a2dafca467e6 100644
--- a/arch/x86/ras/mce_amd_inj.c
+++ b/arch/x86/ras/mce_amd_inj.c
@@ -457,8 +457,12 @@ static int __init init_mce_inject(void)
 						    &i_mce,
 						    dfs_fls[i].fops);
 
-		if (!dfs_fls[i].d)
+		if (!dfs_fls[i].d) {
+			if (!i)
+				return -ENODEV;
+
 			goto err_dfs_add;
+		}
 	}
 
 	return 0;
@@ -470,7 +474,7 @@ err_dfs_add:
 	debugfs_remove(dfs_inj);
 	dfs_inj = NULL;
 
-	return -ENOMEM;
+	return -ENODEV;
 }
 
 static void __exit exit_mce_inject(void)

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--