Mailing List Archive

On Mon, Jan 29, 2007 at 12:33:28PM +0100, Heiko Carstens wrote:
> On Wed, Jan 24, 2007 at 01:09:18AM -0800, akpm@osdl.org wrote:
> > mm-search_binary_handler-mem-limit-fix.patch
> > From: Dmitriy Monakhov <dmonakhov@openvz.org>
> >
> > The function changes mem limit to USER_DS before possible modprobe, but
> > never restored it again.
> >
> > Signed-off-by: Dmitriy Monakhov <dmonakhov@openvz.org>
> > Signed-off-by: Andrew Morton <akpm@osdl.org>
> > fs/exec.c | 10 +++++++---
> > 1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff -puN fs/exec.c~mm-search_binary_handler-mem-limit-fix fs/exec.c
> > --- a/fs/exec.c~mm-search_binary_handler-mem-limit-fix
> > +++ a/fs/exec.c
> > @@ -1018,6 +1018,7 @@ int search_binary_handler(struct linux_b
> > {
> > int try,retval;
> > struct linux_binfmt *fmt;
> > + mm_segment_t oldfs;
> > #ifdef __alpha__
> > /* handle /sbin/loader.. */
> > {
> > @@ -1059,11 +1060,12 @@ int search_binary_handler(struct linux_b
> >
> > /* kernel module loader fixup */
> > /* so we don't try to load run modprobe in kernel space. */
> > + oldfs = get_fs();
> > set_fs(USER_DS);
> >
> > retval = audit_bprm(bprm);
> > if (retval)
> > - return retval;
> > + goto out;
> >
> > retval = -ENOENT;
> > for (try=0; try<2; try++) {
> > @@ -1084,7 +1086,7 @@ int search_binary_handler(struct linux_b
> > bprm->file = NULL;
> > current->did_exec = 1;
> > proc_exec_connector(current);
> > - return retval;
> > + goto out;
> > }
> > read_lock(&binfmt_lock);
> > put_binfmt(fmt);
> > @@ -1092,7 +1094,7 @@ int search_binary_handler(struct linux_b
> > break;
> > if (!bprm->file) {
> > read_unlock(&binfmt_lock);
> > - return retval;
> > + goto out;
> > }
> > }
> > read_unlock(&binfmt_lock);
> > @@ -1110,6 +1112,8 @@ int search_binary_handler(struct linux_b
> > #endif
> > }
> > }
> > +out:
> > + set_fs(oldfs);
> > return retval;
> > }

This is broken. This is the only place in kernel that sets fs to USER_DS
for a new process. With this patch we could as well get rid of USER_DS and
all the address space checkings.
Besides that it breaks architectures with distinct physical address spaces.
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On Mon, 29 Jan 2007 14:59:58 +0100
Heiko Carstens <heiko.carstens@de.ibm.com> wrote:

> On Mon, Jan 29, 2007 at 12:33:28PM +0100, Heiko Carstens wrote:
> > On Wed, Jan 24, 2007 at 01:09:18AM -0800, akpm@osdl.org wrote:
> > > mm-search_binary_handler-mem-limit-fix.patch
> > > From: Dmitriy Monakhov <dmonakhov@openvz.org>
> > >
> > > The function changes mem limit to USER_DS before possible modprobe, but
> > > never restored it again.
> > >
> > > Signed-off-by: Dmitriy Monakhov <dmonakhov@openvz.org>
> > > Signed-off-by: Andrew Morton <akpm@osdl.org>
> > > fs/exec.c | 10 +++++++---
> > > 1 file changed, 7 insertions(+), 3 deletions(-)
> > >
> > > diff -puN fs/exec.c~mm-search_binary_handler-mem-limit-fix fs/exec.c
> > > --- a/fs/exec.c~mm-search_binary_handler-mem-limit-fix
> > > +++ a/fs/exec.c
> > > @@ -1018,6 +1018,7 @@ int search_binary_handler(struct linux_b
> > > {
> > > int try,retval;
> > > struct linux_binfmt *fmt;
> > > + mm_segment_t oldfs;
> > > #ifdef __alpha__
> > > /* handle /sbin/loader.. */
> > > {
> > > @@ -1059,11 +1060,12 @@ int search_binary_handler(struct linux_b
> > >
> > > /* kernel module loader fixup */
> > > /* so we don't try to load run modprobe in kernel space. */
> > > + oldfs = get_fs();
> > > set_fs(USER_DS);
> > >
> > > retval = audit_bprm(bprm);
> > > if (retval)
> > > - return retval;
> > > + goto out;
> > >
> > > retval = -ENOENT;
> > > for (try=0; try<2; try++) {
> > > @@ -1084,7 +1086,7 @@ int search_binary_handler(struct linux_b
> > > bprm->file = NULL;
> > > current->did_exec = 1;
> > > proc_exec_connector(current);
> > > - return retval;
> > > + goto out;
> > > }
> > > read_lock(&binfmt_lock);
> > > put_binfmt(fmt);
> > > @@ -1092,7 +1094,7 @@ int search_binary_handler(struct linux_b
> > > break;
> > > if (!bprm->file) {
> > > read_unlock(&binfmt_lock);
> > > - return retval;
> > > + goto out;
> > > }
> > > }
> > > read_unlock(&binfmt_lock);
> > > @@ -1110,6 +1112,8 @@ int search_binary_handler(struct linux_b
> > > #endif
> > > }
> > > }
> > > +out:
> > > + set_fs(oldfs);
> > > return retval;
> > > }
>
> This is broken. This is the only place in kernel that sets fs to USER_DS
> for a new process. With this patch we could as well get rid of USER_DS and
> all the address space checkings.
> Besides that it breaks architectures with distinct physical address spaces.

hm, thanks for testing - I'll drop it.

I don't really understand what's wrong with it though. Maybe it's settng
USER_DS on kernel threads?

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On Mon, 2007-01-29 at 09:37 -0800, Andrew Morton wrote:
> hm, thanks for testing - I'll drop it.
>
> I don't really understand what's wrong with it though. Maybe it's settng
> USER_DS on kernel threads?

For architectures with a split address space there has to be a call
set_fs(USER_DS) that switches from KERNEL_DS to USER_DS for the init
process. So far this has been done in search_binary_handler and
traditionally the kernel starts with KERNEL_DS to make the early
copy_from_user calls work.
So, what is wrong with always setting USER_DS? We are starting a user
space process after all.

--
blue skies,
Martin.

Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH

"Reality continues to ruin my life." - Calvin.


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> > > The function changes mem limit to USER_DS before possible modprobe, but
> > > never restored it again.
Truly. The road to hell is paved with good intentions.

Martin Schwidefsky <schwidefsky@de.ibm.com> writes:
> On Mon, 2007-01-29 at 09:37 -0800, Andrew Morton wrote:
>> hm, thanks for testing - I'll drop it.
>>
>> I don't really understand what's wrong with it though. Maybe it's settng
>> USER_DS on kernel threads?
>
> For architectures with a split address space there has to be a call
> set_fs(USER_DS) that switches from KERNEL_DS to USER_DS for the init
> process. So far this has been done in search_binary_handler and
> traditionally the kernel starts with KERNEL_DS to make the early
> copy_from_user calls work.
> So, what is wrong with always setting USER_DS? We are starting a user
> space process after all.
May be add some comment to prevent future attempts to make this place
more "correct"?
>
> --
> blue skies,
> Martin.
>
> Martin Schwidefsky
> Linux for zSeries Development & Services
> IBM Deutschland Entwicklung GmbH
>
> "Reality continues to ruin my life." - Calvin.
>
>
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/

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On Tue, 2007-01-30 at 08:40 +0300, Dmitriy Monakhov wrote:
> > > > The function changes mem limit to USER_DS before possible modprobe, but
> > > > never restored it again.
> Truly. The road to hell is paved with good intentions.

:-)

> > For architectures with a split address space there has to be a call
> > set_fs(USER_DS) that switches from KERNEL_DS to USER_DS for the init
> > process. So far this has been done in search_binary_handler and
> > traditionally the kernel starts with KERNEL_DS to make the early
> > copy_from_user calls work.
> > So, what is wrong with always setting USER_DS? We are starting a user
> > space process after all.
> May be add some comment to prevent future attempts to make this place
> more "correct"?

The use of set_fs(USER_DS) in search_binary_handler is certainly
different compared to the rest. It probably is the only one that is not
paired with a set_fs(KERNEL_DS) or set_fs(old_fs). A comment won't hurt.

--
blue skies,
Martin.

Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH

"Reality continues to ruin my life." - Calvin.


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