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Date: Fri, 23 Aug 2019 07:11:07 -0400
From: Lev Olshvang <levonshe@...dex.com>
To: kernel-hardening@...ts.openwall.com
Subject: [RFC] Refuse write to read_only pages of a target process. Target process is not a current process. It is foreign process. Typically  debuggers,such as gdb,  write to read-only code[text] sections of target. kernel hardening configuration option will stop attacks modifying code or jump tables. New logic denies to accept page fault caused by page protection violation.

Separatly applied for x86,powerpc and unicore32
arch_vma_access_permitted() function is not referenced in unicore32 and um
architectures and seems to be obsolete,IMHO.

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@...dex.com>
---
 arch/powerpc/include/asm/mmu_context.h   |  7 +++++++
 arch/powerpc/mm/book3s64/pkeys.c         |  6 ++++++
 arch/um/include/asm/mmu_context.h        |  6 ++++++
 arch/unicore32/include/asm/mmu_context.h |  8 +++++++-
 arch/x86/include/asm/mmu_context.h       | 10 +++++++++-
 include/asm-generic/mm_hooks.h           |  6 ++++++
 security/Kconfig                         | 11 +++++++++++
 7 files changed, 52 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
index 58efca9..db37c61 100644
--- a/arch/powerpc/include/asm/mmu_context.h
+++ b/arch/powerpc/include/asm/mmu_context.h
@@ -251,6 +251,13 @@ void arch_dup_pkeys(struct mm_struct *oldmm, 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
+	if (write && foreign && (!(vma->vm_flags & VM_WRITE))) {
+		/* Forbid write to PROT_READ pages of foreign process */
+		return false;
+	}
+#endif
+
 	/* by default, allow everything */
 	return true;
 }
diff --git a/arch/powerpc/mm/book3s64/pkeys.c b/arch/powerpc/mm/book3s64/pkeys.c
index ae7fca4..b70fdfd 100644
--- a/arch/powerpc/mm/book3s64/pkeys.c
+++ b/arch/powerpc/mm/book3s64/pkeys.c
@@ -406,6 +406,12 @@ static inline bool vma_is_foreign(struct vm_area_struct *vma)
 bool arch_vma_access_permitted(struct vm_area_struct *vma, bool write,
 			       bool execute, bool foreign)
 {
+#ifdef CONFIG_PROTECT_READONLY_USER_MEMORY
+	if (write && foreign && (!(vma->vm_flags & VM_WRITE))) {
+		/* Forbid write to PROT_READ pages of foreign process */
+		return false;
+	}
+#endif
 	if (static_branch_likely(&pkey_disabled))
 		return true;
 	/*
diff --git a/arch/um/include/asm/mmu_context.h b/arch/um/include/asm/mmu_context.h
index 00cefd3..2c56ce9 100644
--- a/arch/um/include/asm/mmu_context.h
+++ b/arch/um/include/asm/mmu_context.h
@@ -33,6 +33,12 @@ static inline void arch_bprm_mm_init(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
+	if (write && foreign && (!(vma->vm_flags & VM_WRITE))) {
+		/* Forbid write to PROT_READ pages of foreign process */
+		return false;
+	}
+#endif
 	/* by default, allow everything */
 	return true;
 }
diff --git a/arch/unicore32/include/asm/mmu_context.h b/arch/unicore32/include/asm/mmu_context.h
index 247a07a..730997c 100644
--- a/arch/unicore32/include/asm/mmu_context.h
+++ b/arch/unicore32/include/asm/mmu_context.h
@@ -97,7 +97,13 @@ static inline void arch_bprm_mm_init(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
+	if (write && foreign && (!(vma->vm_flags & VM_WRITE))) {
+		/* Forbid write to PROT_READ pages of foreign process */
+		return false;
+	}
+#endif
 	/* by default, allow everything */
 	return true;
 }
-#endif
+#endif /*__UNICORE_MMU_CONTEXT_H__*/
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 9024236..77b2801 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -329,12 +329,20 @@ static inline bool vma_is_foreign(struct vm_area_struct *vma)
 static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
 		bool write, bool execute, bool foreign)
 {
-	/* pkeys never affect instruction fetches */
+
+#ifdef CONFIG_PROTECT_READONLY_USER_MEMORY
+	if (write && foreign && (!(vma->vm_flags & VM_WRITE))) {
+		/* Forbid write to PROT_READ pages of foreign process */
+		return false;
+	}
+#endif
+	/* Don't check PKRU since pkeys never affect instruction fetches */
 	if (execute)
 		return true;
 	/* allow access if the VMA is not one from this process */
 	if (foreign || vma_is_foreign(vma))
 		return true;
+
 	return __pkru_allows_pkey(vma_pkey(vma), write);
 }
 
diff --git a/include/asm-generic/mm_hooks.h b/include/asm-generic/mm_hooks.h
index 6736ed2..31dae5a 100644
--- a/include/asm-generic/mm_hooks.h
+++ b/include/asm-generic/mm_hooks.h
@@ -30,6 +30,12 @@ static inline void arch_bprm_mm_init(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
+	if (write && foreign && (!(vma->vm_flags & VM_WRITE))) {
+		/* Forbid write to PROT_READ pages of foreign process */
+		return false;
+	}
+#endif
 	/* by default, allow everything */
 	return true;
 }
diff --git a/security/Kconfig b/security/Kconfig
index 0d65594..03ff948 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -143,6 +143,17 @@ 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"
+	depends on !(CONFIG_CROSS_MEMORY_ATTACH)
+	help
+	  Protects read only memory of process code and PLT table from possible attack
+	  through /proc/PID/mem.
+	  Forbid writes to READ ONLY user pages of foreign process
+	  Mostly advised for embedded and production system.
+	  Disables process_vm_writev() syscall used in MP computing.
+
+
 config HAVE_HARDENED_USERCOPY_ALLOCATOR
 	bool
 	help
-- 
2.7.4

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