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Date: Wed, 28 Jul 2021 14:52:40 +0800
From: Wang Zi-cheng <wzc@...il.nju.edu.cn>
To: keescook@...omium.org, tycho@...ho.pizza,
linux-hardening@...r.kernel.org
Cc: Wang Zi-cheng <wzc@...il.nju.edu.cn>
Subject: [PATCH] RFC v2 struct const ops pointers member hardening
Signed-off-by: Wang Zi-cheng <wzc@...il.nju.edu.cn>
--------------------
RFC v2 struct const ops pointers member hardening
I apologize for making mistakes in my previous submission, I fix the previous problems,
add compile options and make tests(compile and lmbench).
1. this is a useful hardening, my opinion was wrong in the previous patch,
because the attacker may overwrite a struct with an "struct file*" pointer,
which point to a manufactured file struct with malicious f_op.
Hardening operation pointers CAN help.
> On the other side, kernel uses `kmem_cache_create` to alloc file/inode rather than `kmalloc`,
> which makes it hard to exploit through heap overflow or UAF, so maybe this is not a "must" update.
2. V2 add compile option 'STRUCT_CONST_OPS_POINTER_HARDENING' in
'Security options -> Kernel hardening options -> Struct const operation pointers hardening'
> PATCH v1
> some security sensitive kernel objects, i.e. inode, file, use 'const' to declare operations pointers,
> and these pointers reference to the static global variables in read only data segment.
> ```
> struct file {
> ...
> const struct file_operations *f_op;
>
> const struct file_operations ext4_file_operations = {
> ```
> However, const pointers are just compile hints with no hardware restrictions and prone to be modified by attakers at runtime.
> It would be safer to make sure struct const pointer members are not pointing to malicious payload in the slab objects(direct mapping region).
> we only need to check "open" syscall, because most file-related operations start with "open".
---
arch/x86/include/asm/pgtable_64_types.h | 5 +++++
fs/open.c | 5 +++++
include/linux/mm.h | 9 +++++++++
security/Kconfig.hardening | 13 +++++++++++++
4 files changed, 32 insertions(+)
diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
index 91ac10654570..869ef3dfaf29 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -128,18 +128,23 @@ extern unsigned int ptrs_per_p4d;
#define __VMEMMAP_BASE_L4 0xffffea0000000000UL
#define __VMEMMAP_BASE_L5 0xffd4000000000000UL
+#define DIRECT_MAPPING_TB_L4 64UL
+#define DIRECT_MAPPING_TB_L5 32000UL
#ifdef CONFIG_DYNAMIC_MEMORY_LAYOUT
# define VMALLOC_START vmalloc_base
# define VMALLOC_SIZE_TB (pgtable_l5_enabled() ? VMALLOC_SIZE_TB_L5 : VMALLOC_SIZE_TB_L4)
# define VMEMMAP_START vmemmap_base
+# define DIRECT_MAPPING_SIZE_TB (pgtable_l5_enabled() ? DIRECT_MAPPING_TB_L5 : DIRECT_MAPPING_TB_L4)
#else
# define VMALLOC_START __VMALLOC_BASE_L4
# define VMALLOC_SIZE_TB VMALLOC_SIZE_TB_L4
# define VMEMMAP_START __VMEMMAP_BASE_L4
+# define DIRECT_MAPPING_SIZE_TB DIRECT_MAPPING_TB_L4
#endif /* CONFIG_DYNAMIC_MEMORY_LAYOUT */
#define VMALLOC_END (VMALLOC_START + (VMALLOC_SIZE_TB << 40) - 1)
+#define DIRECT_MAPPING_END (__PAGE_OFFSET + (DIRECT_MAPPING_SIZE_TB << 40) - 1)
#define MODULES_VADDR (__START_KERNEL_map + KERNEL_IMAGE_SIZE)
/* The module sections ends with the start of the fixmap */
diff --git a/fs/open.c b/fs/open.c
index e53af13b5835..871750b57267 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -820,6 +820,11 @@ static int do_dentry_open(struct file *f,
/* normally all 3 are set; ->open() can clear them if needed */
f->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
+ /* check if f_op point to malicious payload */
+#ifdef CONFIG_STRUCT_CONST_OPS_POINTER_HARDENING
+ WARN_ON (!check_valid_ops_pointer((unsigned long) f->f_op));
+ WARN_ON (!check_valid_ops_pointer((unsigned long) f->f_inode->i_op));
+#endif
if (!open)
open = f->f_op->open;
if (open) {
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8ae31622deef..c787acda7012 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3250,6 +3250,15 @@ static inline int seal_check_future_write(int seals, struct vm_area_struct *vma)
return 0;
}
+#ifdef CONFIG_STRUCT_CONST_OPS_POINTER_HARDENING
+static inline bool check_valid_ops_pointer(unsigned long addr)
+{
+ if (addr >= __PAGE_OFFSET && addr <= DIRECT_MAPPING_END)
+ return false;
+ else
+ return true;
+}
+#endif
#endif /* __KERNEL__ */
#endif /* _LINUX_MM_H */
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index a56c36470cb1..3a7e2a046842 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -219,4 +219,17 @@ config INIT_ON_FREE_DEFAULT_ON
endmenu
+config STRUCT_CONST_OPS_POINTER_HARDENING
+ depends on X86_64
+ bool "Struct const operation pointers hardening"
+ help
+ Security sensitive kernel objects, i.e. 'inode', 'file' protect
+ indirect call pointers by declaring const operation pointers and
+ making these pointers reference to static const global variables.
+ However const members in the kernel object are just compile hints
+ with no hardware restriction. To hardening the operations pointers
+ in structs, put a check in "open syscall" and make sure pointers
+ are not pointing to the malicious payload in slub
+ objects(direct mapping region).
+
endmenu
--
2.32.0
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