lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Tue, 22 Jun 2021 11:51:51 -0700
From:   Nadav Amit <>
To:     Matthew Wilcox <>
Cc:     Linus Torvalds <>,
        David Howells <>,
        Al Viro <>, Ted Ts'o <>,
        Dave Hansen <>,
        Andrew Morton <>,
        Linux-MM <>,
        Ext4 Developers List <>,
        linux-fsdevel <>,
        Linux Kernel Mailing List <>
Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user
 buffer pages"?

> On Jun 22, 2021, at 11:36 AM, Matthew Wilcox <> wrote:
> On Tue, Jun 22, 2021 at 11:28:30AM -0700, Linus Torvalds wrote:
>> On Tue, Jun 22, 2021 at 11:23 AM Matthew Wilcox <> wrote:
>>> It wouldn't be _that_ bad necessarily.  filemap_fault:
>> It's not actually the mm code that is the biggest problem. We
>> obviously already have readahead support.
>> It's the *fault* side.
>> In particular, since the fault would return without actually filling
>> in the page table entry (because the page isn't ready yet, and you
>> cannot expose it to other threads!), you also have to jump over the
>> instruction that caused this all.
> Oh, I was assuming that it'd be a function call like
> get_user_pages_fast(), not an instruction that was specially marked to
> be jumped over.  Gag reflex diminishing now?

Just reminding the alternative (in the RFC that I mentioned before):
a vDSO exception table entry for a memory accessing function in the
vDSO. It then behaves as a sort of MADV_WILLNEED for the faulting
page if an exception is triggered. Unlike MADV_WILLNEED it maps the
page if no IO is needed. It can return through a register whether
the page was present or not.

I once implemented (another) alternative, in which the ELF had a section
with an exception-table (holding all the “Async-#PF” instructions),
which described where to skip to if a #PF occurs, but this solution
seemed too heavy-weight/intrusive.

Powered by blists - more mailing lists