| Message ID | 20210322120935.391973-1-ben.dooks@codethink.co.uk (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | riscv: evaluate put_user() argument before enabling user access | expand |
On Mär 22 2021, Ben Dooks wrote: > + * to the result of dereferencing @ptr. The @x is copied inside the macro The value of @x > + * to avoid code re-ordering where @x gets evaulated within the block that > + * enables user-space access (thus possibly bypassing some of the protection > + * this feautre provides). feature Andreas.
On 22/03/2021 13:12, Andreas Schwab wrote: > On Mär 22 2021, Ben Dooks wrote: > >> + * to the result of dereferencing @ptr. The @x is copied inside the macro > The value of @x > >> + * to avoid code re-ordering where @x gets evaulated within the block that >> + * enables user-space access (thus possibly bypassing some of the protection >> + * this feautre provides). > feature Thanks, will fix this for the next version. Do people think we should get this included into stable?
diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h index 824b2c9da75b..7bf90d462ec9 100644 --- a/arch/riscv/include/asm/uaccess.h +++ b/arch/riscv/include/asm/uaccess.h @@ -306,7 +306,10 @@ 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 @x is copied inside the macro + * to avoid code re-ordering where @x gets evaulated within the block that + * enables user-space access (thus possibly bypassing some of the protection + * this feautre provides). * * Caller must check the pointer with access_ok() before calling this * function. @@ -316,12 +319,13 @@ 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; \
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 assinging '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 Cc: dvyukov@google.com Cc: linux-riscv@lists.infradead.org Cc: paul.walmsley@sifive.com Cc: palmer@dabbelt.com Cc: alex@ghiti.fr Cc: linux-kernel@lists.codethink.co.uk Cc: hch@infradead.org --- arch/riscv/include/asm/uaccess.h | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)