Re: randomized placement of x86_64 vdso

[Nathan Lynch]

On 04/23/2014 11:30 AM, H. Peter Anvin wrote:
> On 04/21/2014 09:52 AM, Nathan Lynch wrote:
>> Hi x86/vdso people,
>>
>> I've been working on adding a vDSO to 32-bit ARM, and Kees suggested I
>> look at x86_64's algorithm for placing the vDSO at a randomized offset
>> above the stack VMA.  I found that when the stack top occupies the
>> last slot in the PTE (is that the right term?), the vdso_addr routine
>> returns an address below mm->start_stack, equivalent to
>> (mm->start_stack & PAGE_MASK).  For instance if mm->start_stack is
>> 0x7fff3ffffc96, vdso_addr returns 0x7fff3ffff000.
>>
>> Since the address returned is always already occupied by the stack,
>> get_unmapped_area detects the collision and falls back to
>> vm_unmapped_area.  This results in the vdso being placed in the
>> address space next to libraries etc.  While this is generally
>> unnoticeable and doesn't break anything, it does mean that the vdso is
>> placed below the stack when there is actually room above the stack.
>> To me it also seems uncomfortably close to placing the vdso in the way
>> of downward expansion of the stack.
>>
>> I don't have a patch because I'm not sure what the algorithm should
>> be, but thought I would bring it up as vdso_addr doesn't seem to be
>> behaving as intended in all cases.
>>
> 
> If the stack occupies the last possible page, how can you say there is
> "space above the stack"?

Sorry for being unclear.  I probably am getting terminology wrong.  What
I'm trying to express is that if the stack top is in the last page of
its last-level page table (which may be the last possible page, but
that's not really the interesting case), vdso_addr returns an address
below mm->start_stack.

If you do a lot of execs with the following debug patch applied,
you should see occasional prints like:

got addr 0x7f9a2ba16000, asked 0x7fffa7bff000, start_stack=0x7fffa7bffc96
got addr 0x7f3877ff1000, asked 0x7fffd9bff000, start_stack=0x7fffd9bffc96
got addr 0x7f96e3637000, asked 0x7ffff39ff000, start_stack=0x7ffff39ffc96
got addr 0x7fb70588d000, asked 0x7fff271ff000, start_stack=0x7fff271ffc96
got addr 0x7f7957171000, asked 0x7fff71dff000, start_stack=0x7fff71dffc96

Hopefully this better illustrates.

diff --git a/arch/x86/vdso/vma.c b/arch/x86/vdso/vma.c
index 1ad102613127..06c51329d1b3 100644
--- a/arch/x86/vdso/vma.c
+++ b/arch/x86/vdso/vma.c
@@ -157,15 +157,17 @@ static int setup_additional_pages(struct linux_binprm *bprm,
 				  unsigned size)
 {
 	struct mm_struct *mm = current->mm;
-	unsigned long addr;
+	unsigned long addr, hint;
 	int ret;
 
 	if (!vdso_enabled)
 		return 0;
 
 	down_write(&mm->mmap_sem);
-	addr = vdso_addr(mm->start_stack, size);
-	addr = get_unmapped_area(NULL, addr, size, 0, 0);
+	hint = vdso_addr(mm->start_stack, size);
+	addr = get_unmapped_area(NULL, hint, size, 0, 0);
+	if (addr != hint)
+		pr_info("got addr 0x%lx, asked 0x%lx\n", addr, hint);
 	if (IS_ERR_VALUE(addr)) {
 		ret = addr;
 		goto up_fail;

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