Commit d1d7f01f authored by Johannes Berg's avatar Johannes Berg
Browse files

um: mark rodata read-only and implement _nofault accesses 



Mark read-only data actually read-only (simple mprotect), and
to be able to test it also implement _nofault accesses. This
works by setting up a new "segv_continue" pointer in current,
and then when we hit a segfault we change the signal return
context so that we continue at that address. The code using
this sets it up so that it jumps to a label and then aborts
the access that way, returning -EFAULT.

It's possible to optimize the ___backtrack_faulted() thing by
using asm goto (compiler version dependent) and/or gcc's (not
sure if clang has it) &&label extension, but at least in one
attempt I made the && caused the compiler to not load -EFAULT
into the register in case of jumping to the &&label from the
fault handler. So leave it like this for now.

Signed-off-by: default avatarJohannes Berg <johannes.berg@intel.com>
Co-developed-by: default avatarBenjamin Berg <benjamin.berg@intel.com>
Signed-off-by: default avatarBenjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250210160926.420133-2-benjamin@sipsolutions.net


Signed-off-by: default avatarJohannes Berg <johannes.berg@intel.com>
parent 5550187c

arch/um/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -12,6 +12,7 @@ config UML 
	select ARCH_HAS_KCOV

	select ARCH_HAS_STRNCPY_FROM_USER

	select ARCH_HAS_STRNLEN_USER

	select ARCH_HAS_STRICT_KERNEL_RWX

	select HAVE_ARCH_AUDITSYSCALL

	select HAVE_ARCH_KASAN if X86_64

	select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN

arch/um/include/asm/processor-generic.h

+2 −0
Original line number Diff line number Diff line
@@ -31,6 +31,8 @@ struct thread_struct { 
		} thread;

	} request;



	void *segv_continue;



	/* Contains variable sized FP registers */

	struct pt_regs regs;

};

arch/um/include/asm/uaccess.h

+15 −5
Original line number Diff line number Diff line
@@ -9,6 +9,7 @@ 


#include <asm/elf.h>

#include <linux/unaligned.h>

#include <sysdep/faultinfo.h>



#define __under_task_size(addr, size) \

	(((unsigned long) (addr) < TASK_SIZE) && \
@@ -44,19 +45,28 @@ static inline int __access_ok(const void __user *ptr, unsigned long size) 
		 __access_ok_vsyscall(addr, size));

}



/* no pagefaults for kernel addresses in um */

#define __get_kernel_nofault(dst, src, type, err_label)			\

do {									\

	*((type *)dst) = get_unaligned((type *)(src));			\

	if (0) /* make sure the label looks used to the compiler */	\

	int __faulted;							\

									\

	___backtrack_faulted(__faulted);				\

	if (__faulted) {						\

		*((type *)dst) = (type) 0;				\

		goto err_label;						\

	}								\

	*((type *)dst) = get_unaligned((type *)(src));			\

	current->thread.segv_continue = NULL;				\

} while (0)



#define __put_kernel_nofault(dst, src, type, err_label)			\

do {									\

	put_unaligned(*((type *)src), (type *)(dst));			\

	if (0) /* make sure the label looks used to the compiler */	\

	int __faulted;							\

									\

	___backtrack_faulted(__faulted);				\

	if (__faulted)							\

		goto err_label;						\

	put_unaligned(*((type *)src), (type *)(dst));			\

	current->thread.segv_continue = NULL;				\

} while (0)



#endif

arch/um/include/shared/arch.h

+2 −0
Original line number Diff line number Diff line
@@ -12,4 +12,6 @@ extern void arch_check_bugs(void); 
extern int arch_fixup(unsigned long address, struct uml_pt_regs *regs);

extern void arch_examine_signal(int sig, struct uml_pt_regs *regs);



void mc_set_rip(void *_mc, void *target);



#endif

arch/um/include/shared/as-layout.h

+1 −1
Original line number Diff line number Diff line
@@ -50,7 +50,7 @@ extern int linux_main(int argc, char **argv, char **envp); 
extern void uml_finishsetup(void);



struct siginfo;

extern void (*sig_info[])(int, struct siginfo *si, struct uml_pt_regs *);

extern void (*sig_info[])(int, struct siginfo *si, struct uml_pt_regs *, void *);



#endif