Re: [PATCH] mm: mmap() sometimes succeeds even if the region to mapis invalid.
From: Naotaka Hamaguchi
Date: Tue Feb 21 2012 - 03:30:09 EST
Hi Hugh,
1. Should a negative offset necessarily return -EINVAL? At present I
can mmap() /dev/kmem on x86_64 and see what's at 0xffff880000000000:
why should that say -EINVAL? (I admit that my example wanted to say
0xffffffff81000000, where /proc/kallsyms locates _text, but that did
disappoint me with -EINVAL, because mmap_kmem() only understands the
direct map, not the further layouts which architectures may use.)
2. We will have bugs if you manage to mmap an area crossing from pgoff
-1 to pgoff 0, but I thought the existing checks prevented that.
- if ((pgoff + (len>> PAGE_SHIFT))< pgoff)
+ if ((off + len)< off)
return -EOVERFLOW;
I think you are taking away the 32-bit kernel's ability to mmap() files
up to MAX_LFS_FILESIZE.
Thanks, I see. I drop this patch.
BTW, I think the current error check of EOVERFLOW is meaningless, isn't it?
mm/mmap.c
===================================================================
unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
unsigned long len, unsigned long prot,
unsigned long flags, unsigned long pgoff)
{
...
/* offset overflow? */
if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
return -EOVERFLOW;
...
===================================================================
Thanks,
Naotaka Hamaguchi
(2012/02/18 11:00), Hugh Dickins wrote:
On Fri, 17 Feb 2012, Naotaka Hamaguchi wrote:
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.
I'm not convinced that either of these is a problem. Do you see an
actual bug arising from these, or is it just that you think the Linux
mmap() permits more than you expect from your reading of POSIX?
1. Should a negative offset necessarily return -EINVAL? At present I
can mmap() /dev/kmem on x86_64 and see what's at 0xffff880000000000:
why should that say -EINVAL? (I admit that my example wanted to say
0xffffffff81000000, where /proc/kallsyms locates _text, but that did
disappoint me with -EINVAL, because mmap_kmem() only understands the
direct map, not the further layouts which architectures may use.)
2. We will have bugs if you manage to mmap an area crossing from pgoff
-1 to pgoff 0, but I thought the existing checks prevented that.
mmap() should be permitting as far as it safely can; but it's a bug
if a fault on an offset beyond (page-rounded-up) end-of-file does not
then give SIGBUS.
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@xxxxxxxxxxxxxx>
---
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;
I think you are taking away the 32-bit kernel's ability to mmap() files
up to MAX_LFS_FILESIZE.
Hugh
/* Too many mappings? */
--
1.7.7.4
Best Regards,
Naotaka Hamaguchi
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