Date: Thu, 8 Feb 2018 14:18:04 +1100 From: "Tobin C. Harding" <me@...in.cc> To: Matthew Wilcox <willy@...radead.org> Cc: linux-mm@...ck.org, kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> Subject: Re: [RFC] Warn the user when they could overflow mapcount On Wed, Feb 07, 2018 at 06:11:12PM -0800, Matthew Wilcox wrote: > > Kirill and I were talking about trying to overflow page->_mapcount > the other day and realised that the default settings of pid_max and > max_map_count prevent it . But there isn't even documentation to > warn a sysadmin that they've just opened themselves up to the possibility > that they've opened their system up to a sufficiently-determined attacker. > > I'm not sufficiently wise in the ways of the MM to understand exactly > what goes wrong if we do wrap mapcount. Kirill says: > > rmap depends on mapcount to decide when the page is not longer mapped. > If it sees page_mapcount() == 0 due to 32-bit wrap we are screwed; > data corruption, etc. > > That seems pretty bad. So here's a patch which adds documentation to the > two sysctls that a sysadmin could use to shoot themselves in the foot, > and adds a warning if they change either of them to a dangerous value. > It's possible to get into a dangerous situation without triggering this > warning (already have the file mapped a lot of times, then lower pid_max, > then raise max_map_count, then map the file a lot more times), but it's > unlikely to happen. > > Comments? > >  map_count counts the number of times that a page is mapped to > userspace; max_map_count restricts the number of times a process can > map a page and pid_max restricts the number of processes that can exist. > So map_count can never be larger than pid_max * max_map_count. > > diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt > index 412314eebda6..ec90cd633e99 100644 > --- a/Documentation/sysctl/kernel.txt > +++ b/Documentation/sysctl/kernel.txt > @@ -718,6 +718,8 @@ pid_max: > PID allocation wrap value. When the kernel's next PID value > reaches this value, it wraps back to a minimum PID value. > PIDs of value pid_max or larger are not allocated. > +Increasing this value without decreasing vm.max_map_count may > +allow a hostile user to corrupt kernel memory > > ============================================================== > > diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt > index ff234d229cbb..0ab306ea8f80 100644 > --- a/Documentation/sysctl/vm.txt > +++ b/Documentation/sysctl/vm.txt > @@ -379,7 +379,8 @@ While most applications need less than a thousand maps, certain > programs, particularly malloc debuggers, may consume lots of them, > e.g., up to one or two maps per allocation. > > -The default value is 65536. > +The default value is 65530. Increasing this value without decreasing > +pid_max may allow a hostile user to corrupt kernel memory. Just checking - did you mean the final '0' on this value? Tobin
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