linux-kernel - [PATCH] x86/mm: only allow memmap=XX!YY over existing RAM

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Message-Id: <1466408039-7497-1-git-send-email-yigal@plexistor.com>
Date:	Mon, 20 Jun 2016 10:33:59 +0300
From:	Yigal Korman <yigal@...xistor.com>
To:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	linux-nvdimm@...ts.01.org, dan.j.williams@...el.com,
	elliott@....com, jmoyer@...hat.com, toshi.kani@....com,
	Yigal Korman <yigal@...xistor.com>
Subject: [PATCH] x86/mm: only allow memmap=XX!YY over existing RAM

Before this patch, passing a range that is beyond the physical memory
range will succeed, the user will see a /dev/pmem0 and will be able to
access it. Reads will always return 0 and writes will be silently
ignored.

I've gotten more than one bug report about mkfs.{xfs,ext4} or nvml
failing that were eventually tracked down to be wrong values passed to
memmap.

This patch prevents the above issue by instead of adding a new memory
range, only update a RAM memory range with the PRAM type. This way,
passing the wrong memmap will either not give you a pmem at all or give
you a smaller one that actually has RAM behind it.

And if someone still needs to fake a pmem that doesn't have RAM behind
it, they can simply do memmap=XX@YY,XX!YY.

Signed-off-by: Yigal Korman <yigal@...xistor.com>
---
 arch/x86/kernel/e820.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 621b501..4bd4207 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -878,7 +878,7 @@ static int __init parse_memmap_one(char *p)
 		e820_add_region(start_at, mem_size, E820_RESERVED);
 	} else if (*p == '!') {
 		start_at = memparse(p+1, &p);
-		e820_add_region(start_at, mem_size, E820_PRAM);
+		e820_update_range(start_at, mem_size, E820_RAM, E820_PRAM);
 	} else
 		e820_remove_range(mem_size, ULLONG_MAX - mem_size, E820_RAM, 1);

-- 
1.9.3