linux-kernel - [PATCH] mm: mmap() sometimes succeeds even if the region to map is invalid.

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Date:	Fri, 17 Feb 2012 17:43:05 +0900
From:	Naotaka Hamaguchi <n.hamaguchi@...fujitsu.com>
To:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Subject: [PATCH] mm: mmap() sometimes succeeds even if the region to map is
 invalid.

This patch fixes two bugs of mmap():
 1. mmap() succeeds even if "offset" argument is a negative value, although
    it should return EINVAL in such case. Currently I have only checked
    it on x86_64 because (a) x86 seems to OK to accept a negative offset
    for mapping 2GB-4GB regions, and (b) I don't know about other
    architectures at all (I'll make it if needed).

 2. mmap() would succeed if "offset" + "length" get overflow, although
    it should return EOVERFLOW.

The detail of these problems is as follows:

1. mmap() succeeds even if "offset" argument is a negative value, although
   it should return EINVAL in such case.

POSIX says the type of the argument "off" is "off_t", which
is equivalent to "long" for all architecture, so it is allowed to
give a negative "off" to mmap().

In such case, it is actually regarded as big positive value
because the type of "off" is "unsigned long" in the kernel. 
For example, off=-4096 (-0x1000) is regarded as 
off = 0xfffffffffffff000 (x86_64) and as off = 0xfffff000 (x86).
It results in mapping too big offset region.

2. mmap() would succeed if "offset" + "length" get overflow, although
   it should return EOVERFLOW.

The overflow check of mmap() almost doesn't work.

In do_mmap_pgoff(file, addr, len, prot, flags, pgoff),
the existing overflow check logic is as follows.

------------------------------------------------------------------------
do_mmap_pgoff(struct file *file, unsigned long addr,
		unsigned long len, unsigned long prot,
		unsigned long flags, unsigned long pgoff)
{
	if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
		return -EOVERFLOW;
}
------------------------------------------------------------------------

However, for example on x86_64, if we give off=0x1000 and
len=0xfffffffffffff000, but EOVERFLOW is not returned.
It is because the checking is based on the page offset,
not on the byte offset.

To fix this bug, I convert this overflow check from page
offset base to byte offset base. 

Signed-off-by: Naotaka Hamaguchi <n.hamaguchi@...fujitsu.com>
---
 arch/x86/kernel/sys_x86_64.c |    3 +++
 mm/mmap.c                    |    3 ++-
 2 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
index 0514890..ddefd6c 100644
--- a/arch/x86/kernel/sys_x86_64.c
+++ b/arch/x86/kernel/sys_x86_64.c
@@ -90,6 +90,9 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
        if (off & ~PAGE_MASK)
                goto out;

+       if ((off_t) off < 0)
+               goto out;
+
        error = sys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
 out:
        return error;
diff --git a/mm/mmap.c b/mm/mmap.c
index 3f758c7..2fa99cd 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -948,6 +948,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
        vm_flags_t vm_flags;
        int error;
        unsigned long reqprot = prot;
+       unsigned long off = pgoff << PAGE_SHIFT;

        /*
         * Does the application expect PROT_READ to imply PROT_EXEC?
@@ -971,7 +972,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
                return -ENOMEM;

        /* offset overflow? */
-       if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
+       if ((off + len) < off)
                return -EOVERFLOW;

        /* Too many mappings? */
--
1.7.7.4

Best Regards,
Naotaka Hamaguchi

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