[PATCH v2 4/4] bpf: export bpf_jit_current

[PATCH v2 4/4] bpf: export bpf_jit_current

[Lorenz Bauer]

Expose bpf_jit_current as a read only value via sysctl.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
---
 Documentation/admin-guide/sysctl/net.rst |  6 ++++++
 include/linux/filter.h                   |  1 +
 kernel/bpf/core.c                        |  3 +--
 net/core/sysctl_net_core.c               | 24 ++++++++++++++++++++++++
 4 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/net.rst b/Documentation/admin-guide/sysctl/net.rst
index 4150f74c521a..524e7db8d53f 100644
--- a/Documentation/admin-guide/sysctl/net.rst
+++ b/Documentation/admin-guide/sysctl/net.rst
@@ -123,6 +123,12 @@ compiler in order to reject unprivileged JIT requests once it has
 been surpassed. bpf_jit_limit contains the value of the global limit
 in bytes.
 
+bpf_jit_current
+---------------
+
+The amount of JIT memory currently allocated, in bytes. JITing of
+unprivileged BPF is rejected if this value is above bpf_jit_limit.
+
 dev_weight
 ----------
 
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 8231a6a257f6..42c543a21cd8 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1051,6 +1051,7 @@ extern int bpf_jit_harden;
 extern int bpf_jit_kallsyms;
 extern long bpf_jit_limit;
 extern long bpf_jit_limit_max;
+extern atomic_long_t bpf_jit_current;
 
 typedef void (*bpf_jit_fill_hole_t)(void *area, unsigned int size);
 
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index ab84b3816339..12aedab09222 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -526,6 +526,7 @@ int bpf_jit_kallsyms __read_mostly = IS_BUILTIN(CONFIG_BPF_JIT_DEFAULT_ON);
 int bpf_jit_harden   __read_mostly;
 long bpf_jit_limit   __read_mostly;
 long bpf_jit_limit_max __read_mostly;
+atomic_long_t bpf_jit_current __read_mostly;
 
 static void
 bpf_prog_ksym_set_addr(struct bpf_prog *prog)
@@ -801,8 +802,6 @@ int bpf_jit_add_poke_descriptor(struct bpf_prog *prog,
 	return slot;
 }
 
-static atomic_long_t bpf_jit_current;
-
 /* Can be overridden by an arch's JIT compiler if it has a custom,
  * dedicated BPF backend memory area, or if neither of the two
  * below apply.
diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
index 5f88526ad61c..78603f561482 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -15,6 +15,7 @@
 #include <linux/vmalloc.h>
 #include <linux/init.h>
 #include <linux/slab.h>
+#include <linux/atomic.h>
 
 #include <net/ip.h>
 #include <net/sock.h>
@@ -307,6 +308,22 @@ proc_dolongvec_minmax_bpf_restricted(struct ctl_table *table, int write,
 
 	return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 }
+
+static int proc_bpf_jit_current(struct ctl_table *table, int write,
+				void *buffer, size_t *lenp, loff_t *ppos)
+{
+	long curr = atomic_long_read(&bpf_jit_current) << PAGE_SHIFT;
+	struct ctl_table ctl_entry = {
+		.data		= &curr,
+		.maxlen		= sizeof(long),
+	};
+
+
+	if (!capable(CAP_SYS_ADMIN) || write)
+		return -EPERM;
+
+	return proc_doulongvec_minmax(&ctl_entry, write, buffer, lenp, ppos);
+}
 #endif
 
 static struct ctl_table net_core_table[] = {
@@ -421,6 +438,13 @@ static struct ctl_table net_core_table[] = {
 		.extra1		= &long_one,
 		.extra2		= &bpf_jit_limit_max,
 	},
+	{
+		.procname	= "bpf_jit_current",
+		.data		= &bpf_jit_current,
+		.maxlen		= sizeof(long),
+		.mode		= 0400,
+		.proc_handler	= proc_bpf_jit_current,
+	},
 #endif
 	{
 		.procname	= "netdev_tstamp_prequeue",
-- 
2.30.2

Re: [PATCH v2 4/4] bpf: export bpf_jit_current

[Nicolas Dichtel]

Le 12/10/2021 à 15:59, Lorenz Bauer a écrit :
> Expose bpf_jit_current as a read only value via sysctl.
> 
> Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
> ---

[snip]

> +	{
> +		.procname	= "bpf_jit_current",
> +		.data		= &bpf_jit_current,
> +		.maxlen		= sizeof(long),
> +		.mode		= 0400,
Why not 0444 ?


Regards,
Nicolas

Re: [PATCH v2 4/4] bpf: export bpf_jit_current

[Lorenz Bauer]

On Tue, 12 Oct 2021 at 17:29, Nicolas Dichtel <nicolas.dichtel@6wind.com> wrote:
>
> Le 12/10/2021 à 15:59, Lorenz Bauer a écrit :
> > Expose bpf_jit_current as a read only value via sysctl.
> >
> > Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
> > ---
>
> [snip]
>
> > +     {
> > +             .procname       = "bpf_jit_current",
> > +             .data           = &bpf_jit_current,
> > +             .maxlen         = sizeof(long),
> > +             .mode           = 0400,
> Why not 0444 ?

This mirrors what the other BPF related sysctls do, which only allow
access from root with CAP_SYS_ADMIN. I'd prefer 0444 as well, but
Daniel explicitly locked down these sysctls in
2e4a30983b0f9b19b59e38bbf7427d7fdd480d98.

Lorenz

--
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

www.cloudflare.com

Re: [PATCH v2 4/4] bpf: export bpf_jit_current

[Nicolas Dichtel]

Le 13/10/2021 à 10:35, Lorenz Bauer a écrit :
> On Tue, 12 Oct 2021 at 17:29, Nicolas Dichtel <nicolas.dichtel@6wind.com> wrote:
>>
>> Le 12/10/2021 à 15:59, Lorenz Bauer a écrit :
>>> Expose bpf_jit_current as a read only value via sysctl.
>>>
>>> Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
>>> ---
>>
>> [snip]
>>
>>> +     {
>>> +             .procname       = "bpf_jit_current",
>>> +             .data           = &bpf_jit_current,
>>> +             .maxlen         = sizeof(long),
>>> +             .mode           = 0400,
>> Why not 0444 ?
> 
> This mirrors what the other BPF related sysctls do, which only allow
> access from root with CAP_SYS_ADMIN. I'd prefer 0444 as well, but
> Daniel explicitly locked down these sysctls in
> 2e4a30983b0f9b19b59e38bbf7427d7fdd480d98.
Even after this patch, bpf_jit_enable is 0644.

In fact, if you have CAP_BPF or CAP_SYS_ADMIN, this value has no impact for your
programs. But I you don't have one of these capabilities, it may be rejected,
but you cannot read these values, which help to understand why.


Regards,
Nicolas