[PATCH v2 1/9] s390/uaccess: Add storage key checked cmpxchg access to user space

Janis Schoetterl-Glausch
2022-10-12T20:56:42Z

Add cmpxchg functionality similar to that in cmpxchg.h except that the
target is a user space address and that the address' storage key is
matched with the access_key argument in order to honor key-controlled
protection.
The access is performed by changing to the secondary-spaces mode and
setting the PSW key for the duration of the compare and swap.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
---

Possible variations:
* check the assumptions made in cmpxchg_user_key_size and error out
* call functions called by copy_to_user
* access_ok? is a nop
* should_fail_usercopy?
* instrument_copy_to_user? doesn't make sense IMO
* don't be overly strict in cmpxchg_user_key

arch/s390/include/asm/uaccess.h | 189 ++++++++++++++++++++++++++++++++
1 file changed, 189 insertions(+)

diff --git a/arch/s390/include/asm/uaccess.h b/arch/s390/include/asm/uaccess.h
index f7038b800cc3..f148f5a22c93 100644
--- a/arch/s390/include/asm/uaccess.h
+++ b/arch/s390/include/asm/uaccess.h
@@ -19,6 +19,8 @@
#include <asm/extable.h>
#include <asm/facility.h>
#include <asm-generic/access_ok.h>
+#include <asm/page.h>
+#include <linux/log2.h>

void debug_user_asce(int exit);

@@ -390,4 +392,191 @@ do { \
goto err_label; \
} while (0)

+static __always_inline int __cmpxchg_user_key_small(int size, u64 address,
+ unsigned __int128 *old_p,
+ unsigned __int128 new, u8 access_key)
+{
+ u32 shift, mask, old_word, new_word, align_mask, tmp;
+ u64 aligned;
+ int ret = -EFAULT;
+
+ switch (size) {
+ case 2:
+ align_mask = 2;
+ aligned = (address ^ (address & align_mask));
+ shift = (sizeof(u32) - (address & align_mask) - size) * 8;
+ mask = 0xffff << shift;
+ old_word = ((u16)*old_p) << shift;
+ new_word = ((u16)new) << shift;
+ break;
+ case 1:
+ align_mask = 3;
+ aligned = (address ^ (address & align_mask));
+ shift = (sizeof(u32) - (address & align_mask) - size) * 8;
+ mask = 0xff << shift;
+ old_word = ((u8)*old_p) << shift;
+ new_word = ((u8)new) << shift;
+ break;
+ }
+ tmp = old_word; /* don't modify *old_p on fault */
+ asm volatile(
+ "spka 0(%[access_key])\n"
+ " sacf 256\n"
+ "0: l %[tmp],%[aligned]\n"
+ "1: nr %[tmp],%[mask]\n"
+ " xilf %[mask],0xffffffff\n"
+ " or %[new_word],%[tmp]\n"
+ " or %[tmp],%[old_word]\n"
+ "2: lr %[old_word],%[tmp]\n"
+ "3: cs %[tmp],%[new_word],%[aligned]\n"
+ "4: jnl 5f\n"
+ /* We'll restore old_word before the cs, use reg for the diff */
+ " xr %[old_word],%[tmp]\n"
+ /* Apply diff assuming only bits outside target byte(s) changed */
+ " xr %[new_word],%[old_word]\n"
+ /* If prior assumption false we exit loop, so not an issue */
+ " nr %[old_word],%[mask]\n"
+ " jz 2b\n"
+ "5: ipm %[ret]\n"
+ " srl %[ret],28\n"
+ "6: sacf 768\n"
+ " spka %[default_key]\n"
+ EX_TABLE(0b, 6b) EX_TABLE(1b, 6b)
+ EX_TABLE(3b, 6b) EX_TABLE(4b, 6b)
+ : [old_word] "+&d" (old_word),
+ [new_word] "+&d" (new_word),
+ [tmp] "+&d" (tmp),
+ [aligned] "+Q" (*(u32 *)aligned),
+ [ret] "+d" (ret)
+ : [access_key] "a" (access_key << 4),
+ [mask] "d" (~mask),
+ [default_key] "J" (PAGE_DEFAULT_KEY)
+ : "cc"
+ );
+ *old_p = (tmp & mask) >> shift;
+ return ret;
+}
+
+/**
+ * cmpxchg_user_key_size() - cmpxchg with user space target, honoring storage keys
+ * @size: Size of the value being cmpxchg'ed, one of 1,2,4,8,16.
+ * @address: User space address of value to compare to *@old_p and exchange with
+ * @new. Must be aligned to @size.
+ * @old_p: Pointer to old value. Interpreted as a @size byte integer and compared
+ * to the content pointed to by @address in order to determine if the
+ * exchange occurs. The value read from @address is written back to *@old_p.
+ * @new: New value to place at @address, interpreted as a @size byte integer.
+ * @access_key: Access key to use for checking storage key protection.
+ *
+ * Perform a cmpxchg on a user space target, honoring storage key protection.
+ * @access_key alone determines how key checking is performed, neither
+ * storage-protection-override nor fetch-protection-override apply.
+ *
+ * Return: 0: successful exchange
+ * 1: exchange failed
+ * -EFAULT: @address not accessible or not naturally aligned
+ * -EINVAL: invalid @size
+ */
+static __always_inline int cmpxchg_user_key_size(int size, void __user *address,
+ unsigned __int128 *old_p,
+ unsigned __int128 new, u8 access_key)
+{
+ union {
+ u32 word;
+ u64 doubleword;
+ } old;
+ int ret = -EFAULT;
+
+ /*
+ * The following assumes that:
+ * * the current psw key is the default key
+ * * no storage protection overrides are in effect
+ */
+ might_fault();
+ switch (size) {
+ case 16:
+ asm volatile(
+ "spka 0(%[access_key])\n"
+ " sacf 256\n"
+ "0: cdsg %[old],%[new],%[target]\n"
+ "1: ipm %[ret]\n"
+ " srl %[ret],28\n"
+ "2: sacf 768\n"
+ " spka %[default_key]\n"
+ EX_TABLE(0b, 2b) EX_TABLE(1b, 2b)
+ : [old] "+d" (*old_p),
+ [target] "+Q" (*(unsigned __int128 __user *)address),
+ [ret] "+d" (ret)
+ : [access_key] "a" (access_key << 4),
+ [new] "d" (new),
+ [default_key] "J" (PAGE_DEFAULT_KEY)
+ : "cc"
+ );
+ return ret;
+ case 8:
+ old.doubleword = *old_p;
+ asm volatile(
+ "spka 0(%[access_key])\n"
+ " sacf 256\n"
+ "0: csg %[old],%[new],%[target]\n"
+ "1: ipm %[ret]\n"
+ " srl %[ret],28\n"
+ "2: sacf 768\n"
+ " spka %[default_key]\n"
+ EX_TABLE(0b, 2b) EX_TABLE(1b, 2b)
+ : [old] "+d" (old.doubleword),
+ [target] "+Q" (*(u64 __user *)address),
+ [ret] "+d" (ret)
+ : [access_key] "a" (access_key << 4),
+ [new] "d" ((u64)new),
+ [default_key] "J" (PAGE_DEFAULT_KEY)
+ : "cc"
+ );
+ *old_p = old.doubleword;
+ return ret;
+ case 4:
+ old.word = *old_p;
+ asm volatile(
+ "spka 0(%[access_key])\n"
+ " sacf 256\n"
+ "0: cs %[old],%[new],%[target]\n"
+ "1: ipm %[ret]\n"
+ " srl %[ret],28\n"
+ "2: sacf 768\n"
+ " spka %[default_key]\n"
+ EX_TABLE(0b, 2b) EX_TABLE(1b, 2b)
+ : [old] "+d" (old.word),
+ [target] "+Q" (*(u32 __user *)address),
+ [ret] "+d" (ret)
+ : [access_key] "a" (access_key << 4),
+ [new] "d" ((u32)new),
+ [default_key] "J" (PAGE_DEFAULT_KEY)
+ : "cc"
+ );
+ *old_p = old.word;
+ return ret;
+ case 2:
+ case 1:
+ return __cmpxchg_user_key_small(size, (u64)address, old_p, new, access_key);
+ default:
+ return -EINVAL;
+ }
+}
+
+#define cmpxchg_user_key(target_p, old_p, new, access_key) \
+({ \
+ __typeof__(old_p) __old_p = (old_p); \
+ unsigned __int128 __old = *__old_p; \
+ size_t __size = sizeof(*(target_p)); \
+ int __ret; \
+ \
+ BUILD_BUG_ON(__size != sizeof(*__old_p)); \
+ BUILD_BUG_ON(__size != sizeof(new)); \
+ BUILD_BUG_ON(__size > 16 || !is_power_of_2(__size)); \
+ __ret = cmpxchg_user_key_size(__size, (target_p), &__old, (new), \
+ (access_key)); \
+ *__old_p = __old; \
+ __ret; \
+})
+
#endif /* __S390_UACCESS_H */
--
2.34.1