linux-kernel - [PATCH] /dev/mem: remove superfluous error

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Date:	Fri, 8 Jul 2016 11:38:28 +0200 (CEST)
From:	Jiri Kosina <jikos@...nel.org>
To:	x86@...nel.org, Andrew Morton <akpm@...ux-foundation.org>
cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] /dev/mem: remove superfluous error

From: Jiri Kosina <jkosina@...e.cz>

Currently it's possible for broken (or malicious) userspace to flood a 
kernel log indefinitely with messages a-la

	Program dmidecode tried to access /dev/mem between f0000->100000

because range_is_allowed() is case of CONFIG_STRICT_DEVMEM being turned on 
dumps this information each and every time devmem_is_allowed() fails.

Reportedly userspace that is able to trigger contignuous flow of these 
messages exists.

It would be possible to rate limit this message, but that'd have a 
questionable value; the administrator wouldn't get information about all 
the failing accessess, so then the information would be both superfluous 
and incomplete at the same time :)

Returning EPERM (which is what is actually happening) is enough indication 
for userspace what has happened; no need to log this particular error as 
some sort of special condition.

Signed-off-by: Jiri Kosina <jkosina@...e.cz>
---
 arch/x86/mm/pat.c  | 5 +----
 drivers/char/mem.c | 6 +-----
 2 files changed, 2 insertions(+), 9 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index fb0604f..db00e3e 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -755,11 +755,8 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
 		return 1;
 
 	while (cursor < to) {
-		if (!devmem_is_allowed(pfn)) {
-			pr_info("x86/PAT: Program %s tried to access /dev/mem between [mem %#010Lx-%#010Lx], PAT prevents it\n",
-				current->comm, from, to - 1);
+		if (!devmem_is_allowed(pfn))
 			return 0;
-		}
 		cursor += PAGE_SIZE;
 		pfn++;
 	}
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 71025c2..d633974 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -66,12 +66,8 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
 	u64 cursor = from;
 
 	while (cursor < to) {
-		if (!devmem_is_allowed(pfn)) {
-			printk(KERN_INFO
-		"Program %s tried to access /dev/mem between %Lx->%Lx.\n",
-				current->comm, from, to);
+		if (!devmem_is_allowed(pfn))
 			return 0;
-		}
 		cursor += PAGE_SIZE;
 		pfn++;
 	}

-- 
Jiri Kosina
SUSE Labs