[PATCH] ARM: fix get_user() broken with veneer

[Masahiro Yamada <masahiroy@kernel.org>]

The 32-bit ARM kernel stops working when it exceeds a certain size
threshold.

This problem can occur, for instance, when enabling certain debug CONFIG
options or when linking numerous drivers as builtins, but reproducing
it primarily depends on the kernel size. To demonstrate this, I wrote a
debug Makefile code.

The following debug code generates and links drivers/dummy-*.S. The
NR_DUMMY_FILES variable specifies the number of dummy-*.S files to link.
The added code should be harmless except growing the kernel, as it
introduces unused symbols, dummy_0, dummy_1, and so on, each of which
occupies 64K code.

[Applying debug Makefile code]

Run the following code.

$ cat <<'EOF' >> drivers/Makefile

dummy-sources := $(shell i=0; while [ "$$i" -lt "$(NR_DUMMY_FILES)" ]; do echo dummy-$$i.S; i=$$(( i + 1 )); done)
targets += $(dummy-sources)
obj-y += $(patsubst %.S,%.o,$(dummy-sources))

quiet_cmd_gen_dummy = GEN   $@
      cmd_gen_dummy = { \
          echo '\#include <linux/linkage.h>'; \
          echo '.section ".text","ax"'; \
          echo "ENTRY(dummy_$*)"; \
          echo ".fill 65536, 1, 0"; \
          echo "ENDPROC(dummy_$*)"; \
      } > $@

$(obj)/dummy-%.S: FORCE; $(call if_changed,gen_dummy)
EOF

[Build result]

You can easily increase the .text section by giving NR_DUMMY_FILES from
the build command.

 [1] ARM defconfig + NR_DUMMY_FILES=250

  This inserts (64K * 250) dummy code on top of defconfig.
  The kernel boots.

  $ make -j$(nproc) ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- NR_DUMMY_FILES=250 defconfig all
  $ arm-linux-gnueabihf-size vmlinux
  text      data     bss     dec       hex      filename
  33361996  9278130  427088  43067214  291274e  vmlinux
  $ arm-linux-gnueabihf-nm vmlinux | grep ' [tT] ' | grep __get_user_4
  c20e5dbc T __get_user_4

 [2] ARM defconfig + NR_DUMMY_FILES=350

  This inserts (64K * 350) dummy code on top of defconfig.
  The kernel does not boot any more.

  $ make -j$(nproc) ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- NR_DUMMY_FILES=350 defconfig all
  $ arm-linux-gnueabihf-size vmlinux
  text      data     bss     dec       hex      filename
  39946652  9282226  427088  49655966  2f5b09e  vmlinux
  $ arm-linux-gnueabihf-nm vmlinux | grep ' [tT] ' | grep __get_user_4
  c272931c T __get_user_4
  c06f6b10 t ____get_user_4_veneer
  c0aef930 t ____get_user_4_veneer

I noticed ____get_user_4_veneer was causing the issue.

After commit 02e541db0540 ("ARM: 8323/1: force linker to use PIC veneers"),
veneers use the ip (= r12) register.

The disassembly of the veneer looks as follows:

  c06f6b10 <____get_user_4_veneer>:
  c06f6b10:       e59fc000        ldr     ip, [pc]        @ c06f6b18 <____get_user_4_veneer+0x8>
  c06f6b14:       e08ff00c        add     pc, pc, ip
  c06f6b18:       02032800        .word   0x02032800

When a routine is called via a veneer, the ip register is destroyed.

It is absolutely fine.

AAPCS32 [1] says:

 "Register r12 (IP) may be used by a linker as a scratch register between
  a routine and any subroutine it calls. It can also be used within a
  routine to hold intermediate values between subroutine calls."

Of course, the compiler knows it, but the problem here is, the compiler
cannot notice the 'bl' instruction inside the inline asm.

In __get_user_check(), uaccess_save_and_enable() and uaccess_restore()
functions read and restore the value of the DACR register. There is no
obvious function call between them because __get_user_x() is a macro.
However, there is a branch to __get_user_4 buried within inline asm,
which the compiler is not aware of. Because only "lr" and "cc" are
listed in the clobber, the compiler considers it safe to use the ip
register to hold __ua_flags.

The disassembly of do_undefinstr() looks as follows:

  c030b2bc <do_undefinstr>:
    [snip]
  c030b3d8:       e1a01006        mov     r1, r6
  c030b3dc:       e306077c        movw    r0, #26492        @ 0x677c
  c030b3e0:       e34c02c5        movt    r0, #49861        @ 0xc2c5
  c030b3e4:       e3a02000        mov     r2, #0
  c030b3e8:       e8bd41f0        pop     {r4, r5, r6, r7, r8, lr}
  c030b3ec:       eafffe2e        b       c030acac <die>
  c030b3f0:       ee13cf10        mrc     15, 0, ip, cr3, cr0, {0}          <== DACR is loaded to ip
  c030b3f4:       e3cc300c        bic     r3, ip, #12
  c030b3f8:       e1a00007        mov     r0, r7
  c030b3fc:       e3833004        orr     r3, r3, #4
  c030b400:       e3e01441        mvn     r1, #1090519040        @ 0x41000000
  c030b404:       ee033f10        mcr     15, 0, r3, cr3, cr0, {0}
  c030b408:       f57ff06f        isb     sy
  c030b40c:       eb0fadbf        bl      c06f6b10 <____get_user_4_veneer>  <== ip is destroyed here
  c030b410:       ee03cf10        mcr     15, 0, ip, cr3, cr0, {0}          <== ip is written back to DACR
  c030b414:       f57ff06f        isb     sy
  c030b418:       e3500000        cmp     r0, #0

Add "ip" to the clobber list to inform the compiler that it may be
destroyed if we end up with a veneer.

[1]: https://github.com/ARM-software/abi-aa/blob/2023Q1/aapcs32/aapcs32.rst

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

I did not check the entire ARM code to seek for a similar pattern,
but I confirmed  __put_user_check() had "ip" "lr" "cc" in the clobber
list.

Perhaps, another way is to list out all registers not guaranteed
in the spec.

AAPCS32 says, "A subroutine must preserve the contents of the registers
r4-r8, r10, r11 and SP".

The others might be clobbered. Of course, clobbered registers are
predicable as we write the subroutines in assembly code (only ip is
conditionally clobbered if veneers are generated), but it will be
safe in case we change the assembly code.


 arch/arm/include/asm/uaccess.h | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
index bb5c81823117..c28f5ec21e41 100644
--- a/arch/arm/include/asm/uaccess.h
+++ b/arch/arm/include/asm/uaccess.h
@@ -109,16 +109,6 @@ extern int __get_user_64t_1(void *);
 extern int __get_user_64t_2(void *);
 extern int __get_user_64t_4(void *);
 
-#define __GUP_CLOBBER_1	"lr", "cc"
-#ifdef CONFIG_CPU_USE_DOMAINS
-#define __GUP_CLOBBER_2	"ip", "lr", "cc"
-#else
-#define __GUP_CLOBBER_2 "lr", "cc"
-#endif
-#define __GUP_CLOBBER_4	"lr", "cc"
-#define __GUP_CLOBBER_32t_8 "lr", "cc"
-#define __GUP_CLOBBER_8	"lr", "cc"
-
 #define __get_user_x(__r2, __p, __e, __l, __s)				\
 	   __asm__ __volatile__ (					\
 		__asmeq("%0", "r0") __asmeq("%1", "r2")			\
@@ -126,7 +116,7 @@ extern int __get_user_64t_4(void *);
 		"bl	__get_user_" #__s				\
 		: "=&r" (__e), "=r" (__r2)				\
 		: "0" (__p), "r" (__l)					\
-		: __GUP_CLOBBER_##__s)
+		: "ip", "lr", "cc")
 
 /* narrowing a double-word get into a single 32bit word register: */
 #ifdef __ARMEB__
@@ -148,7 +138,7 @@ extern int __get_user_64t_4(void *);
 		"bl	__get_user_64t_" #__s				\
 		: "=&r" (__e), "=r" (__r2)				\
 		: "0" (__p), "r" (__l)					\
-		: __GUP_CLOBBER_##__s)
+		: "ip", "lr", "cc")
 #else
 #define __get_user_x_64t __get_user_x
 #endif
-- 
2.39.2