[RFC PATCH 1/5] security : hardening : prevent write to proces's read-only pages from another process

Lev Olshvang
2020-04-06T14:57:17Z

The purpose of this patch is produce hardened kernel for Embedded
or Production systems.

Typically debuggers, such as gdb, write to read-only code [text]
sections of target process.(ptrace)
This kind of page protectiion violation raises minor page fault, but
kernel's fault handler allows it by default.
This is clearly attack surface for adversary.

The proposed kernel hardening configuration option checks the type of
protection of the foreign vma and blocks writes to read only vma.

When enabled, it will stop attacks modifying code or jump tables, etc.

Code of arch_vma_access_permitted() function was extended to
check foreign vma flags.

Tested on x86_64 and ARM(QEMU) with dd command which writes to
/proc/PID/mem in r--p or r--xp of vma area addresses range

dd reports IO failure when tries to write to adress taken from
from /proc/PID/maps (PLT or code section)

Signed-off-by: Lev Olshvang <levonshe@gmail.com>
---
include/asm-generic/mm_hooks.h | 5 +++++
security/Kconfig | 10 ++++++++++
2 files changed, 15 insertions(+)

diff --git a/include/asm-generic/mm_hooks.h b/include/asm-generic/mm_hooks.h
index 4dbb177d1150..6e1fcce44cc2 100644
--- a/include/asm-generic/mm_hooks.h
+++ b/include/asm-generic/mm_hooks.h
@@ -25,6 +25,11 @@ static inline void arch_unmap(struct mm_struct *mm,
static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
bool write, bool execute, bool foreign)
{
+#ifdef CONFIG_PROTECT_READONLY_USER_MEMORY
+ /* Forbid write to PROT_READ pages of foreign process */
+ if (write && foreign && (!(vma->vm_flags & VM_WRITE)))
+ return false;
+#endif
/* by default, allow everything */
return true;
}
diff --git a/security/Kconfig b/security/Kconfig
index cd3cc7da3a55..d92e79c90d67 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -143,6 +143,16 @@ config LSM_MMAP_MIN_ADDR
this low address space will need the permission specific to the
systems running LSM.

+config PROTECT_READONLY_USER_MEMORY
+ bool "Protect read only process memory"
+ help
+ Protects read only memory of process code and PLT table
+ from possible attack through /proc/PID/mem or through /dev/mem.
+ Refuses to insert and stop at debuggers breakpoints (prtace,gdb)
+ Mostly advised for embedded and production system.
+ Stops attempts of the malicious process to modify read only memory of another process
+
+
config HAVE_HARDENED_USERCOPY_ALLOCATOR
bool
help
--
2.17.1