Messages in this thread | | | Date | Wed, 3 Oct 2012 09:13:45 -0700 | Subject | Re: [PATCH] mm: use %pK for /proc/vmallocinfo | From | Kees Cook <> |
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On Tue, Oct 2, 2012 at 10:37 PM, David Rientjes <rientjes@google.com> wrote: > On Tue, 2 Oct 2012, Kees Cook wrote: > >> >> In the paranoid case of sysctl kernel.kptr_restrict=2, mask the kernel >> >> virtual addresses in /proc/vmallocinfo too. >> >> >> >> Reported-by: Brad Spengler <spender@grsecurity.net> >> >> Signed-off-by: Kees Cook <keescook@chromium.org> >> > >> > /proc/vmallocinfo is S_IRUSR, not S_IRUGO, so exactly what are you trying >> > to protect? >> >> Trying to block the root user from seeing virtual memory addresses >> (mode 2 of kptr_restrict). >> >> Documentation/sysctl/kernel.txt: >> "This toggle indicates whether restrictions are placed on >> exposing kernel addresses via /proc and other interfaces. When >> kptr_restrict is set to (0), there are no restrictions. When >> kptr_restrict is set to (1), the default, kernel pointers >> printed using the %pK format specifier will be replaced with 0's >> unless the user has CAP_SYSLOG. When kptr_restrict is set to >> (2), kernel pointers printed using %pK will be replaced with 0's >> regardless of privileges." >> >> Even though it's S_IRUSR, it still needs %pK for the paranoid case. > > So root does echo 0 > /proc/sys/kernel/kptr_restrict first. Again: what > are you trying to protect?
Only CAP_SYS_ADMIN can change the setting. This is, for example, for containers, or other situations where a uid 0 process lacking CAP_SYS_ADMIN cannot see virtual addresses. It's a very paranoid case, yes, but it's part of how this feature was designed. Think of it as supporting the recent uid 0 vs ring 0 boundary. :)
-Kees
-- Kees Cook Chrome OS Security
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