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 <[email protected]>
---
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
> 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.
> 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.
It is not true always.
Considering your example, say if page size is 4k, then PAGE_MASK = 0xFFF
hence (off & ~PAGE_MASK) will be true and " -EINVAL" will be returned.
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 <[email protected]>
> ---
> 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
Hi Venu,
>> 1. mmap() succeeds even if "offset" argument is a negative value, although
>> it should return EINVAL in such case.
>
>> 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.
>
> It is not true always.
>
> Considering your example, say if page size is 4k, then PAGE_MASK = 0xFFF
> hence (off& ~PAGE_MASK) will be true and " -EINVAL" will be returned.
Is PAGE_MASK 0xfffffffffffff000 (x86_64) and 0xfffff000 (x86), isn't it?
Or am I missing something?
arch/x86/include/asm/page_types.h
=================================================
...
#define PAGE_SHIFT 12
#define PAGE_SIZE (_AC(1,UL) << PAGE_SHIFT)
#define PAGE_MASK (~(PAGE_SIZE-1))
...
=================================================
Thanks,
Naotaka Hamaguchi
(2012/02/17 18:04), Venu Byravarasu wrote:
>> 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.
>
>> 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.
>
> It is not true always.
>
> Considering your example, say if page size is 4k, then PAGE_MASK = 0xFFF
> hence (off& ~PAGE_MASK) will be true and " -EINVAL" will be returned.
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to [email protected]
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
>
>
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<[email protected]>
>> ---
>> 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
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to [email protected]
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
>
>