[PATCH 2/9] ARM: traps: use get_kernel_nofault instead of set_fs()

Arnd Bergmann
2020-10-30T15:49:36Z

From: Arnd Bergmann <arnd@arndb.de>

ARM uses set_fs() and __get_user() to allow the stack dumping code to
access possibly invalid pointers carefully. These can be changed to the
simpler get_kernel_nofault(), and allow the eventual removal of set_fs().

dump_instr() will print either kernel or user space pointers,
depending on how it was called. For dump_mem(), I assume we are only
interested in kernel pointers, and the only time that this is called
with user_mode(regs)==true is when the regs themselves are unreliable
as a result of the condition that caused the trap.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
arch/arm/kernel/traps.c | 47 ++++++++++++++---------------------------
1 file changed, 16 insertions(+), 31 deletions(-)

diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index 17d5a785df28..c3964a283b63 100644
--- a/arch/arm/kernel/traps.c
+++ b/arch/arm/kernel/traps.c
@@ -122,17 +122,8 @@ static void dump_mem(const char *lvl, const char *str, unsigned long bottom,
unsigned long top)
{
unsigned long first;
- mm_segment_t fs;
int i;

- /*
- * We need to switch to kernel mode so that we can use __get_user
- * to safely read from kernel space. Note that we now dump the
- * code first, just in case the backtrace kills us.
- */
- fs = get_fs();
- set_fs(KERNEL_DS);
-
printk("%s%s(0x%08lx to 0x%08lx)\n", lvl, str, bottom, top);

for (first = bottom & ~31; first < top; first += 32) {
@@ -145,7 +136,7 @@ static void dump_mem(const char *lvl, const char *str, unsigned long bottom,
for (p = first, i = 0; i < 8 && p < top; i++, p += 4) {
if (p >= bottom && p < top) {
unsigned long val;
- if (__get_user(val, (unsigned long *)p) == 0)
+ if (get_kernel_nofault(val, (unsigned long *)p))
sprintf(str + i * 9, " %08lx", val);
else
sprintf(str + i * 9, " ????????");
@@ -153,11 +144,9 @@ static void dump_mem(const char *lvl, const char *str, unsigned long bottom,
}
printk("%s%04lx:%s\n", lvl, first & 0xffff, str);
}
-
- set_fs(fs);
}

-static void __dump_instr(const char *lvl, struct pt_regs *regs)
+static void dump_instr(const char *lvl, struct pt_regs *regs)
{
unsigned long addr = instruction_pointer(regs);
const int thumb = thumb_mode(regs);
@@ -173,10 +162,20 @@ static void __dump_instr(const char *lvl, struct pt_regs *regs)
for (i = -4; i < 1 + !!thumb; i++) {
unsigned int val, bad;

- if (thumb)
- bad = get_user(val, &((u16 *)addr)[i]);
- else
- bad = get_user(val, &((u32 *)addr)[i]);
+ if (!user_mode(regs)) {
+ if (thumb) {
+ u16 val16;
+ bad = get_kernel_nofault(val16, &((u16 *)addr)[i]);
+ val = val16;
+ } else {
+ bad = get_kernel_nofault(val, &((u32 *)addr)[i]);
+ }
+ } else {
+ if (thumb)
+ bad = get_user(val, &((u16 *)addr)[i]);
+ else
+ bad = get_user(val, &((u32 *)addr)[i]);
+ }

if (!bad)
p += sprintf(p, i == 0 ? "(%0*x) " : "%0*x ",
@@ -189,20 +188,6 @@ static void __dump_instr(const char *lvl, struct pt_regs *regs)
printk("%sCode: %s\n", lvl, str);
}

-static void dump_instr(const char *lvl, struct pt_regs *regs)
-{
- mm_segment_t fs;
-
- if (!user_mode(regs)) {
- fs = get_fs();
- set_fs(KERNEL_DS);
- __dump_instr(lvl, regs);
- set_fs(fs);
- } else {
- __dump_instr(lvl, regs);
- }
-}
-
#ifdef CONFIG_ARM_UNWIND
static inline void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
const char *loglvl)
--
2.27.0