diff options
authorBen Dooks <ben.dooks@codethink.co.uk>2021-03-29 10:57:49 +0100
committerPalmer Dabbelt <palmerdabbelt@google.com>2021-04-01 21:37:04 -0700
commit285a76bb2cf51b0c74c634f2aaccdb93e1f2a359 (patch)
parent23c1075ae83adaf14ea3f727c40368799f80bccc (diff)
riscv: evaluate put_user() arg before enabling user access
The <asm/uaccess.h> header has a problem with put_user(a, ptr) if the 'a' is not a simple variable, such as a function. This can lead to the compiler producing code as so: 1: enable_user_access() 2: evaluate 'a' into register 'r' 3: put 'r' to 'ptr' 4: disable_user_acess() The issue is that 'a' is now being evaluated with the user memory protections disabled. So we try and force the evaulation by assigning 'x' to __val at the start, and hoping the compiler barriers in enable_user_access() do the job of ordering step 2 before step 1. This has shown up in a bug where 'a' sleeps and thus schedules out and loses the SR_SUM flag. This isn't sufficient to fully fix, but should reduce the window of opportunity. The first instance of this we found is in scheudle_tail() where the code does: $ less -N kernel/sched/core.c 4263 if (current->set_child_tid) 4264 put_user(task_pid_vnr(current), current->set_child_tid); Here, the task_pid_vnr(current) is called within the block that has enabled the user memory access. This can be made worse with KASAN which makes task_pid_vnr() a rather large call with plenty of opportunity to sleep. Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk> Reported-by: syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com Suggested-by: Arnd Bergman <arnd@arndb.de> -- Changes since v1: - fixed formatting and updated the patch description with more info Changes since v2: - fixed commenting on __put_user() (schwab@linux-m68k.org) Change since v3: - fixed RFC in patch title. Should be ready to merge. Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
1 files changed, 5 insertions, 2 deletions
diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
index 824b2c9da75b..f944062c9d99 100644
--- a/arch/riscv/include/asm/uaccess.h
+++ b/arch/riscv/include/asm/uaccess.h
@@ -306,7 +306,9 @@ do { \
* data types like structures or arrays.
* @ptr must have pointer-to-simple-variable type, and @x must be assignable
- * to the result of dereferencing @ptr.
+ * to the result of dereferencing @ptr. The value of @x is copied to avoid
+ * re-ordering where @x is evaluated inside the block that enables user-space
+ * access (thus bypassing user space protection if @x is a function).
* Caller must check the pointer with access_ok() before calling this
* function.
@@ -316,12 +318,13 @@ do { \
#define __put_user(x, ptr) \
({ \
__typeof__(*(ptr)) __user *__gu_ptr = (ptr); \
+ __typeof__(*__gu_ptr) __val = (x); \
long __pu_err = 0; \
__chk_user_ptr(__gu_ptr); \
__enable_user_access(); \
- __put_user_nocheck(x, __gu_ptr, __pu_err); \
+ __put_user_nocheck(__val, __gu_ptr, __pu_err); \
__disable_user_access(); \
__pu_err; \

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