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Date: Fri, 04 Jun 2021 13:15:43 +0300
From: Jarmo Tiitto <jarmo.tiitto@...il.com>
To: Jarmo Tiitto <jarmo.tiitto@...il.com>,
Kees Cook <keescook@...omium.org>
Cc: Sami Tolvanen <samitolvanen@...gle.com>,
Bill Wendling <wcw@...gle.com>,
Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org,
morbo@...gle.com
Subject: Re: [PATCH v2 1/1] pgo: Fix sleep in atomic section in prf_open()
Kees Cook wrote perjantaina 4. kesäkuuta 2021 0.47.23 EEST:
> On Thu, Jun 03, 2021 at 06:53:17PM +0300, Jarmo Tiitto wrote:
> > In prf_open() the required buffer size can be so large that
> > vzalloc() may sleep thus triggering bug:
> >
> > ======
> > BUG: sleeping function called from invalid context at include/linux/sched/mm.h:201
> > in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 337, name: cat
> > CPU: 1 PID: 337 Comm: cat Not tainted 5.13.0-rc2-24-hack+ #154
> > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
> > Call Trace:
> > dump_stack+0xc7/0x134
> > ___might_sleep+0x177/0x190
> > __might_sleep+0x5a/0x90
> > kmem_cache_alloc_node_trace+0x6b/0x3a0
> > ? __get_vm_area_node+0xcd/0x1b0
> > ? dput+0x283/0x300
> > __get_vm_area_node+0xcd/0x1b0
> > __vmalloc_node_range+0x7b/0x420
> > ? prf_open+0x1da/0x580
> > ? prf_open+0x32/0x580
> > ? __llvm_profile_instrument_memop+0x36/0x50
> > vzalloc+0x54/0x60
> > ? prf_open+0x1da/0x580
> > prf_open+0x1da/0x580
> > full_proxy_open+0x211/0x370
> > ....
> > ======
> >
> > Since we can't vzalloc while holding pgo_lock,
> > split the code into steps:
> > * First get buffer size via prf_buffer_size()
> > and release the lock.
> > * Round up to the page size and allocate the buffer.
> > * Finally re-acquire the pgo_lock and call prf_serialize().
> > prf_serialize() will now check if the buffer is large enough
> > and returns -EAGAIN if it is not.
> >
> > New in this v2 patch:
> > The -EAGAIN case was determined to be such rare event that
> > running following in a loop:
> >
> > $cat /sys/kernel/debug/pgo/vmlinux.profraw > vmlinux.profdata;
> >
> > Didn't trigger it, and I don't know if it ever may occur at all.
>
> Hm, I remain nervous that it'll pop up when we least expect it. But, I
> went to go look at this, and I don't understand why we need a lock at
> all for prf_buffer_size(). These appear to be entirely static in size.
>
I would think the reasoning of taking the pgo_lock for prf_buffer_size() is that because
__prf_get_value_size() walks linked lists that are modified by
__llvm_profile_instrument_target() in instrument.c.
However __llvm_profile_instrument_target() looks like it will only append to
llvm_prf_data::values array of llvm_prf_value_node linked lists, so you might be right.
I'll try analyze the prf_buffer_size() better to determine upper bound of memory required,
such that the pgo_lock is absolutely not needed for prf_buffer_size().
> >
> > Signed-off-by: Jarmo Tiitto <jarmo.tiitto@...il.com>
> > ---
> > kernel/pgo/fs.c | 52 ++++++++++++++++++++++++++++++++++++-------------
> > 1 file changed, 38 insertions(+), 14 deletions(-)
> >
> > diff --git a/kernel/pgo/fs.c b/kernel/pgo/fs.c
> > index ef985159dad3..9afd6f001a1b 100644
> > --- a/kernel/pgo/fs.c
> > +++ b/kernel/pgo/fs.c
> > @@ -24,13 +24,14 @@
> > #include <linux/module.h>
> > #include <linux/slab.h>
> > #include <linux/vmalloc.h>
> > +#include <linux/mm.h>
> > #include "pgo.h"
> >
> > static struct dentry *directory;
> >
> > struct prf_private_data {
> > void *buffer;
> > - unsigned long size;
> > + size_t size;
> > };
> >
> > /*
> > @@ -213,6 +214,7 @@ static inline unsigned long prf_get_padding(unsigned long size)
> > return 7 & (sizeof(u64) - size % sizeof(u64));
> > }
> >
> > +/* Note: caller *must* hold pgo_lock */
> > static unsigned long prf_buffer_size(void)
> > {
> > return sizeof(struct llvm_prf_header) +
> > @@ -225,18 +227,21 @@ static unsigned long prf_buffer_size(void)
> >
> > /*
> > * Serialize the profiling data into a format LLVM's tools can understand.
> > + * Note: p->buffer must point into vzalloc()'d
> > + * area of at least prf_buffer_size() in size.
> > * Note: caller *must* hold pgo_lock.
> > */
> > -static int prf_serialize(struct prf_private_data *p)
> > +static int prf_serialize(struct prf_private_data *p, size_t buf_size)
> > {
> > int err = 0;
> > void *buffer;
> >
> > + /* get buffer size, again. */
> > p->size = prf_buffer_size();
> > - p->buffer = vzalloc(p->size);
> >
> > - if (!p->buffer) {
> > - err = -ENOMEM;
> > + /* check for unlikely overflow. */
> > + if (p->size > buf_size) {
> > + err = -EAGAIN;
>
> This can just be ENOMEM instead -- it'll never change in size. (But we
> should absolutely keep the check.)
>
Ok, I was wondering what return value would be appropriate here.
> > goto out;
> > }
> >
> > @@ -259,27 +264,46 @@ static int prf_open(struct inode *inode, struct file *file)
> > {
> > struct prf_private_data *data;
> > unsigned long flags;
> > - int err;
> > + size_t buf_size;
> > + int err = 0;
> >
> > data = kzalloc(sizeof(*data), GFP_KERNEL);
> > if (!data) {
> > err = -ENOMEM;
> > - goto out;
> > + goto out_free;
> > }
> >
> > + /* get buffer size */
> > flags = prf_lock();
> > + buf_size = prf_buffer_size();
> > + prf_unlock(flags);
>
> And there's no locking needed here.
>
> >
> > - err = prf_serialize(data);
> > - if (unlikely(err)) {
> > - kfree(data);
> > - goto out_unlock;
> > + /* allocate, round up to page size. */
> > + buf_size = PAGE_ALIGN(buf_size);
> > + data->buffer = vzalloc(buf_size);
> > +
> > + if (!data->buffer) {
> > + err = -ENOMEM;
> > + goto out_free;
> > }
> >
> > + /* try serialize and get actual
> > + * data length in data->size
> > + */
> > + flags = prf_lock();
> > + err = prf_serialize(data, buf_size);
> > + prf_unlock(flags);
> > +
> > + if (err)
> > + goto out_free;
> > +
> > file->private_data = data;
> > + return 0;
> >
> > -out_unlock:
> > - prf_unlock(flags);
> > -out:
> > +out_free:
> > + if (data)
> > + vfree(data->buffer);
> > + kfree(data);
> > return err;
> > }
> >
> >
> > base-commit: 5d0cda65918279ada060417c5fecb7e86ccb3def
> > --
> > 2.31.1
> >
>
> --
> Kees Cook
>
--
-Jarmo
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