Re: [PATCH] prctl: fix validation of an address

[Andrew Vagin]

On Sun, Dec 30, 2012 at 05:03:07PM -0500, Eric Paris wrote:
> On Sat, 2012-12-29 at 15:00 +0400, Andrey Vagin wrote:
> > The address should be bigger than dac_mmap_min_addr, because
> > a process with CAP_RAWIO can map a vma bellow mmap_min_addr.
> 
> NAK

Currently prctl(PR_SET_MM_*, addr, ) returns EINVAL for valid addresses.
I think it's a bug. Are you agree?

> 
> This doesn't make any sense.  dac_mmap_min_addr should ONLY be used in
> security/min_addr.c and security/commoncap.c.  Period.  You should not

We can add a function to security/commoncap.c...

> be allowed to circumvent LSM protections.  Maybe you are missing that
> mmap_min_addr = max(dac_mmap_min_addr, CONFIG_LSM_MMAP_MIN_ADDR)  ?

Or maybe you are missing ;) that
/*
 * cap_mmap_addr - check if able to map given addr
 * @addr: address attempting to be mapped
 *
 * If the process is attempting to map memory below dac_mmap_min_addr
 * they need  CAP_SYS_RAWIO.

It was a real case. I have an application, which is failed due to
prlctl. This application uses vm.mmap_min_addr to calculate an adress
for a new vma.

$ sysctl -a | grep min_add
vm.mmap_min_addr = 4096

CONFIG_LSM_MMAP_MIN_ADDR could not be got from user space.

This application can use a real value of mmap_min_addr, but it is not
provided into userspace.

> 
> But this patch is absolutely unacceptable.  Maybe you can help me
> understand what problem you had and what you were hoping for?

Currently a task can have user memory area bellow dac_mmap_min_addr,
but prctl returns -EINVAL for such addresses.
How can I understand the reason, if I know that the address is valid?

I like Linux, because I always can predict its behavior, but this
dac_mmap_min_addr vs mmap_min_addr looks stange for me.

> 
> -Eric
> > 
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> > Cc: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
> > Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> > Cc: Eric Paris <eparis@xxxxxxxxxx>
> > Cc: James Morris <jmorris@xxxxxxxxx>
> > Signed-off-by: Andrey Vagin <avagin@xxxxxxxxxx>
> > ---
> >  kernel/sys.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index 265b376..e0e1bbd 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -1868,7 +1868,7 @@ static int prctl_set_mm(int opt, unsigned long addr,
> >  	if (opt == PR_SET_MM_EXE_FILE)
> >  		return prctl_set_mm_exe_file(mm, (unsigned int)addr);
> >  
> > -	if (addr >= TASK_SIZE || addr < mmap_min_addr)
> > +	if (addr >= TASK_SIZE || addr < dac_mmap_min_addr)
> >  		return -EINVAL;
> >  
> >  	error = -EINVAL;
> 
> 
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