[PATCH v7 2/3] ring-buffer: Cap static ring buffer nr_pages

[Vincent Donnefort]

Static ring buffers (i.e. persistent, user-mapped and remote) rely on
the bpage::id field. The number of pages for those ring buffers must fit
into that variable. Enforce this limit on ring buffer creation or
user-mapping.

While at it, make buffer_page::id 31 bits. This does not change the
struct buffer_page size.

Fixes: be68d63a139b ("ring-buffer: Add ring_buffer_alloc_range()")
Signed-off-by: Vincent Donnefort <vdonnef...@google.com>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index ec13779922ff..5e26eeff588f 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -345,7 +345,7 @@ struct buffer_page {
        local_t          entries;       /* entries on this page */
        unsigned long    real_end;      /* real end of data */
        unsigned         order;         /* order of the page */
-       u32              id:30;         /* ID for external mapping */
+       u32              id:31;         /* ID for external mapping */
        u32              range:1;       /* Mapped via a range */
        struct buffer_data_page *page;  /* Actual data page */
 };
@@ -652,6 +652,15 @@ static bool rb_is_static(struct ring_buffer_per_cpu 
*cpu_buffer)
        return cpu_buffer->user_mapped || cpu_buffer->remote || 
cpu_buffer->ring_meta;
 }
 
+static unsigned long rb_static_max_pages(void)
+{
+       /*
+        * Static ring buffers are using bpage::id and must account for the
+        * reader page.
+        */
+       return (1UL << 31) - 1;
+}
+
 struct ring_buffer_iter {
        struct ring_buffer_per_cpu      *cpu_buffer;
        unsigned long                   head;
@@ -2837,6 +2846,10 @@ static struct trace_buffer *alloc_buffer(unsigned long 
size, unsigned flags,
                 */
                nr_pages = (size - sizeof(struct ring_buffer_cpu_meta)) /
                        (subbuf_size + sizeof(int));
+
+               if (nr_pages > rb_static_max_pages())
+                       goto fail_free_buffers;
+
                /* Need at least two pages plus the reader page */
                if (nr_pages < 3)
                        goto fail_free_buffers;
@@ -2869,6 +2882,10 @@ static struct trace_buffer *alloc_buffer(unsigned long 
size, unsigned flags,
                /* The writer is remote. This ring-buffer is read-only */
                atomic_inc(&buffer->record_disabled);
                nr_pages = desc->nr_page_va - 1;
+
+               if (nr_pages > rb_static_max_pages())
+                       goto fail_free_buffers;
+
                if (nr_pages < 2)
                        goto fail_free_buffers;
        } else {
@@ -7834,6 +7851,9 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
        /* prevent another thread from changing buffer/sub-buffer sizes */
        guard(mutex)(&buffer->mutex);
 
+       if (cpu_buffer->nr_pages > rb_static_max_pages())
+               return -E2BIG;
+
        err = rb_alloc_meta_page(cpu_buffer);
        if (err)
                return err;
-- 
2.55.0.691.gc56d675ccc-goog