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SubjectRe: [PATCH] mm: mmap() sometimes succeeds even if the region to map is invalid.
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@jp.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;

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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