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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]


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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/


[Other Archives]     [Linux Kernel Newbies]     [Linux Driver Development]     [Fedora Kernel]     [Linux Kernel Testers]     [Linux SH]     [Linux Omap]     [Linux Kbuild]     [Linux Tape]     [Linux Input]     [Linux Kernel Janitors]     [Linux Kernel Packagers]     [Linux Doc]     [Linux Man Pages]     [Linux API]     [Linux Memory Management]     [Linux Modules]     [Linux Standards]     [Kernel Announce]     [Netdev]     [Git]     [Linux PCI]     Linux CAN Development     [Linux I2C]     [Linux RDMA]     [Linux NUMA]     [Netfilter]     [Netfilter Devel]     [SELinux]     [Bugtraq]     [FIO]     [Linux Perf Users]     [Linux Serial]     [Linux PPP]     [Linux ISDN]     [Linux Next]     [Kernel Stable Commits]     [Linux Tip Commits]     [Kernel MM Commits]     [Linux Security Module]     [Filesystem Development]     [Ext3 Filesystem]     [Linux bcache]     [Ext4 Filesystem]     [Linux BTRFS]     [Linux CEPH Filesystem]     [Linux XFS]     [XFS]     [Linux NFS]     [Linux CIFS]     [Ecryptfs]     [Linux NILFS]     [Linux Cachefs]     [Reiser FS]     [Initramfs]     [Linux FB Devel]     [Linux OpenGL]     [DRI Devel]     [Fastboot]     [Linux RT Users]     [Linux RT Stable]     [eCos]     [Corosync]     [Linux Clusters]     [LVS Devel]     [Hot Plug]     [Linux Virtualization]     [KVM]     [KVM PPC]     [KVM ia64]     [Linux Containers]     [Linux Hexagon]     [Linux Cgroups]     [Util Linux]     [Wireless]     [Linux Bluetooth]     [Bluez Devel]     [Ethernet Bridging]     [Embedded Linux]     [Barebox]     [Linux MMC]     [Linux IIO]     [Sparse]     [Smatch]     [Linux Arch]     [x86 Platform Driver]     [Linux ACPI]     [Linux IBM ACPI]     [LM Sensors]     [CPU Freq]     [Linux Power Management]     [Linmodems]     [Linux DCCP]     [Linux SCTP]     [ALSA Devel]     [Linux USB]     [Linux PA RISC]     [Linux Samsung SOC]     [MIPS Linux]     [IBM S/390 Linux]     [ARM Linux]     [ARM Kernel]     [ARM MSM]     [Tegra Devel]     [Sparc Linux]     [Linux Security]     [Linux Sound]     [Linux Media]     [Video 4 Linux]     [Linux IRDA Users]     [Linux for the blind]     [Linux RAID]     [Linux ATA RAID]     [Device Mapper]     [Linux SCSI]     [SCSI Target Devel]     [Linux SCSI Target Infrastructure]     [Linux IDE]     [Linux SMP]     [Linux AXP]     [Linux Alpha]     [Linux M68K]     [Linux ia64]     [Linux 8086]     [Linux x86_64]     [Linux Config]     [Linux Apps]     [Linux MSDOS]     [Linux X.25]     [Linux Crypto]     [DM Crypt]     [Linux Trace Users]     [Linux Btrace]     [Linux Watchdog]     [Utrace Devel]     [Linux C Programming]     [Linux Assembly]     [Dash]     [DWARVES]     [Hail Devel]     [Linux Kernel Debugger]     [Linux gcc]     [Gcc Help]     [X.Org]     [Wine]

Add to Google Powered by Linux

[Older Kernel Discussion]     [Yosemite National Park Forum]     [Large Format Photos]     [Gimp]     [Yosemite Photos]     [Stuff]