x86/mm: care about shadow stack guard gap during placement

When memory is being placed, mmap() will take care to respect the guard
gaps of certain types of memory (VM_SHADOWSTACK, VM_GROWSUP and
VM_GROWSDOWN).  In order to ensure guard gaps between mappings, mmap()
needs to consider two things:

 1. That the new mapping isn't placed in an any existing mappings guard
    gaps.
 2. That the new mapping isn't placed such that any existing mappings
    are not in *its* guard gaps.

The longstanding behavior of mmap() is to ensure 1, but not take any care
around 2.  So for example, if there is a PAGE_SIZE free area, and a mmap()
with a PAGE_SIZE size, and a type that has a guard gap is being placed,
mmap() may place the shadow stack in the PAGE_SIZE free area.  Then the
mapping that is supposed to have a guard gap will not have a gap to the
adjacent VMA.

Now that the vm_flags is passed into the arch get_unmapped_area()'s, and
vm_unmapped_area() is ready to consider it, have VM_SHADOW_STACK's get
guard gap consideration for scenario 2.

Link: https://lkml.kernel.org/r/20240326021656.202649-14-rick.p.edgecombe@intel.com
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Deepak Gupta <debug@rivosinc.com>
Cc: Guo Ren <guoren@kernel.org>
Cc: Helge Deller <deller@gmx.de>
Cc: H. Peter Anvin (Intel) <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Naveen N. Rao <naveen.n.rao@linux.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
index 75966afb6251bec19fab1ac806c389e80662423f..01d7cd85ef9789d5742ca7e9f7dc7f2f11063fd7 100644 (file)
--- a/arch/x86/kernel/sys_x86_64.c
+++ b/arch/x86/kernel/sys_x86_64.c
@@ -112,6 +112,14 @@ static void find_start_end(unsigned long addr, unsigned long flags,
                *end = task_size_64bit(addr > DEFAULT_MAP_WINDOW);
 }
 
+static inline unsigned long stack_guard_placement(vm_flags_t vm_flags)
+{
+       if (vm_flags & VM_SHADOW_STACK)
+               return PAGE_SIZE;
+
+       return 0;
+}
+
 unsigned long
 arch_get_unmapped_area_vmflags(struct file *filp, unsigned long addr, unsigned long len,
                       unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags)
@@ -141,6 +149,7 @@ arch_get_unmapped_area_vmflags(struct file *filp, unsigned long addr, unsigned l
        info.low_limit = begin;
        info.high_limit = end;
        info.align_offset = pgoff << PAGE_SHIFT;
+       info.start_gap = stack_guard_placement(vm_flags);
        if (filp) {
                info.align_mask = get_align_mask();
                info.align_offset += get_align_bits();
@@ -190,6 +199,7 @@ get_unmapped_area:
                info.low_limit = PAGE_SIZE;
 
        info.high_limit = get_mmap_base(0);
+       info.start_gap = stack_guard_placement(vm_flags);
 
        /*
         * If hint address is above DEFAULT_MAP_WINDOW, look for unmapped area