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Message-Id: <20240215231332.1556787-6-rick.p.edgecombe@intel.com>
Date: Thu, 15 Feb 2024 15:13:29 -0800
From: Rick Edgecombe <rick.p.edgecombe@...el.com>
To: Liam.Howlett@...cle.com,
akpm@...ux-foundation.org,
debug@...osinc.com,
broonie@...nel.org,
kirill.shutemov@...ux.intel.com,
keescook@...omium.org,
tglx@...utronix.de,
mingo@...hat.com,
bp@...en8.de,
dave.hansen@...ux.intel.com,
x86@...nel.org,
luto@...nel.org,
peterz@...radead.org,
hpa@...or.com,
linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Cc: rick.p.edgecombe@...el.com
Subject: [RFC PATCH 5/8] mm: Take placement mappings gap into account
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 long standing 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.
For MAP_GROWSDOWN/VM_GROWSDOWN and MAP_GROWSUP/VM_GROWSUP this has not
been a problem in practice because applications place these kinds of
mappings very early, when there is not many mappings to find a space
between. But for shadow stacks, they may be placed throughout the lifetime
of the application.
So define a VM_UNMAPPED_START_GAP_SET flag to specify that a start_gap
field has been set, as most vm_unmapped_area_info structs are not zeroed,
so the added field will often contain garbage. Use
VM_UNMAPPED_START_GAP_SET in unmapped_area/_topdown() to find a space that
includes the guard gap for the new mapping. Take care to not interfere
with the alignment.
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
---
include/linux/mm.h | 2 ++
mm/mmap.c | 21 ++++++++++++++-------
2 files changed, 16 insertions(+), 7 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9addf16dbf18..160bb6db7a16 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3393,12 +3393,14 @@ extern unsigned long __must_check vm_mmap(struct file *, unsigned long,
struct vm_unmapped_area_info {
#define VM_UNMAPPED_AREA_TOPDOWN 1
+#define VM_UNMAPPED_START_GAP_SET 2
unsigned long flags;
unsigned long length;
unsigned long low_limit;
unsigned long high_limit;
unsigned long align_mask;
unsigned long align_offset;
+ unsigned long start_gap;
};
extern unsigned long vm_unmapped_area(struct vm_unmapped_area_info *info);
diff --git a/mm/mmap.c b/mm/mmap.c
index 936d728ba1ca..1b6c333656f9 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1567,14 +1567,17 @@ static inline int accountable_mapping(struct file *file, vm_flags_t vm_flags)
*/
static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
{
- unsigned long length, gap;
+ unsigned long length, gap, start_gap = 0;
unsigned long low_limit, high_limit;
struct vm_area_struct *tmp;
MA_STATE(mas, ¤t->mm->mm_mt, 0, 0);
+ if (info->flags & VM_UNMAPPED_START_GAP_SET)
+ start_gap = info->start_gap;
+
/* Adjust search length to account for worst case alignment overhead */
- length = info->length + info->align_mask;
+ length = info->length + info->align_mask + start_gap;
if (length < info->length)
return -ENOMEM;
@@ -1586,7 +1589,7 @@ static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
if (mas_empty_area(&mas, low_limit, high_limit - 1, length))
return -ENOMEM;
- gap = mas.index;
+ gap = mas.index + start_gap;
gap += (info->align_offset - gap) & info->align_mask;
tmp = mas_next(&mas, ULONG_MAX);
if (tmp && (tmp->vm_flags & VM_STARTGAP_FLAGS)) { /* Avoid prev check if possible */
@@ -1619,13 +1622,17 @@ static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
*/
static unsigned long unmapped_area_topdown(struct vm_unmapped_area_info *info)
{
- unsigned long length, gap, gap_end;
+ unsigned long length, gap, gap_end, start_gap = 0;
unsigned long low_limit, high_limit;
struct vm_area_struct *tmp;
MA_STATE(mas, ¤t->mm->mm_mt, 0, 0);
+
+ if (info->flags & VM_UNMAPPED_START_GAP_SET)
+ start_gap = info->start_gap;
+
/* Adjust search length to account for worst case alignment overhead */
- length = info->length + info->align_mask;
+ length = info->length + info->align_mask + start_gap;
if (length < info->length)
return -ENOMEM;
@@ -1832,7 +1839,7 @@ unsigned long mm_get_unmapped_area_vmflags(struct mm_struct *mm, struct file *fi
unsigned long
__get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
- unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags)
+ unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags)
{
unsigned long (*get_area)(struct file *, unsigned long,
unsigned long, unsigned long, unsigned long)
@@ -1883,7 +1890,7 @@ __get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
unsigned long
get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
- unsigned long pgoff, unsigned long flags)
+ unsigned long pgoff, unsigned long flags)
{
return __get_unmapped_area(file, addr, len, pgoff, flags, 0);
}
--
2.34.1
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