On Thu, Mar 10, 2016 at 11:51 AM, Hector Marco-Gisbert <[email protected]> wrote: > Currently on i386 and on X86_64 when emulating X86_32 in legacy mode, only > the stack and the executable are randomized but not other mmapped files > (libraries, vDSO, etc.). This patch enables randomization for the > libraries, vDSO and mmap requests on i386 and in X86_32 in legacy mode. > > By default on i386 there are 8 bits for the randomization of the libraries, > vDSO and mmaps which only uses 1MB of VA. > > This patch preserves the original randomness, using 1MB of VA out of 3GB or > 4GB. We think that 1MB out of 3GB is not a big cost for having the ASLR. > > The first obvious security benefit is that all objects are randomized (not > only the stack and the executable) in legacy mode which highly increases > the ASLR effectiveness, otherwise the attackers may use these > non-randomized areas. But also sensitive setuid/setgid applications are > more secure because currently, attackers can disable the randomization of > these applications by setting the ulimit stack to "unlimited". This is a > very old and widely known trick to disable the ASLR in i386 which has been > allowed for too long. > > Another trick used to disable the ASLR was to set the ADDR_NO_RANDOMIZE > personality flag, but fortunately this doesn't work on setuid/setgid > applications because there is security checks which clear Security-relevant > flags. > > This patch always randomizes the mmap_legacy_base address, removing the > possibility to disable the ASLR by setting the stack to "unlimited". > > > Signed-off-by: Hector Marco-Gisbert <[email protected]> > Signed-off-by: Ismael Ripoll Ripoll <[email protected]>
Thanks! This has bugged me for a long time. :) Acked-by: Kees Cook <[email protected]> Arjan, or other folks, can you remember why x86_32 disabled mmap randomization here? There doesn't seem to be a good reason for it that I see. -Kees > --- > arch/x86/mm/mmap.c | 14 +------------- > 1 file changed, 1 insertion(+), 13 deletions(-) > > diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c > index 72bb52f..d2dc043 100644 > --- a/arch/x86/mm/mmap.c > +++ b/arch/x86/mm/mmap.c > @@ -94,18 +94,6 @@ static unsigned long mmap_base(unsigned long rnd) > } > > /* > - * Bottom-up (legacy) layout on X86_32 did not support randomization, X86_64 > - * does, but not when emulating X86_32 > - */ > -static unsigned long mmap_legacy_base(unsigned long rnd) > -{ > - if (mmap_is_ia32()) > - return TASK_UNMAPPED_BASE; > - else > - return TASK_UNMAPPED_BASE + rnd; > -} > - > -/* > * This function, called very early during the creation of a new > * process VM image, sets up which VM layout function to use: > */ > @@ -116,7 +104,7 @@ void arch_pick_mmap_layout(struct mm_struct *mm) > if (current->flags & PF_RANDOMIZE) > random_factor = arch_mmap_rnd(); > > - mm->mmap_legacy_base = mmap_legacy_base(random_factor); > + mm->mmap_legacy_base = TASK_UNMAPPED_BASE + random_factor; > > if (mmap_is_legacy()) { > mm->mmap_base = mm->mmap_legacy_base; > -- > 1.9.1 > -- Kees Cook Chrome OS & Brillo Security
