On Wed, Jan 4, 2023 at 1:34 AM Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote: > > > On 03/01/2023 23:49, Rob Clark wrote: > > From: Rob Clark <robdclark@xxxxxxxxxxxx> > > > > gem_context_register() makes the context visible to userspace, and which > > point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl. > > So we need to ensure that nothing uses the ctx ptr after this. And we > > need to ensure that adding the ctx to the xarray is the *last* thing > > that gem_context_register() does with the ctx pointer. > > Any backtraces from oopses or notes on how it was found to record in the commit message? It was a UAF bug that was reported to us https://bugs.chromium.org/p/chromium/issues/detail?id=1401594 (but I guess security bugs are not going to be visible) > > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx> > > Fixes: a4c1cdd34e2c ("drm/i915/gem: Delay context creation (v3)") > References: 3aa9945a528e ("drm/i915: Separate GEM context construction and registration to userspace") > Cc: <stable@xxxxxxxxxxxxxxx> # v5.15+ > > > --- > > drivers/gpu/drm/i915/gem/i915_gem_context.c | 24 +++++++++++++++------ > > 1 file changed, 18 insertions(+), 6 deletions(-) > > > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c > > index 7f2831efc798..6250de9b9196 100644 > > --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c > > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c > > @@ -1688,6 +1688,10 @@ void i915_gem_init__contexts(struct drm_i915_private *i915) > > init_contexts(&i915->gem.contexts); > > } > > > > +/* > > + * Note that this implicitly consumes the ctx reference, by placing > > + * the ctx in the context_xa. > > + */ > > static void gem_context_register(struct i915_gem_context *ctx, > > struct drm_i915_file_private *fpriv, > > u32 id) > > @@ -1703,10 +1707,6 @@ static void gem_context_register(struct i915_gem_context *ctx, > > snprintf(ctx->name, sizeof(ctx->name), "%s[%d]", > > current->comm, pid_nr(ctx->pid)); > > > > - /* And finally expose ourselves to userspace via the idr */ > > - old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL); > > - WARN_ON(old); > > - > > spin_lock(&ctx->client->ctx_lock); > > list_add_tail_rcu(&ctx->client_link, &ctx->client->ctx_list); > > spin_unlock(&ctx->client->ctx_lock); > > @@ -1714,6 +1714,10 @@ static void gem_context_register(struct i915_gem_context *ctx, > > spin_lock(&i915->gem.contexts.lock); > > list_add_tail(&ctx->link, &i915->gem.contexts.list); > > spin_unlock(&i915->gem.contexts.lock); > > + > > + /* And finally expose ourselves to userspace via the idr */ > > + old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL); > > + WARN_ON(old); > > Have you seen that this hunk is needed or just moving it for a good measure? To be clear, it is probably best to move it even if the current placement cannot cause any problems, I am just double-checking if you had any concrete observations here while mulling over easier stable backports if we would omit it. > This was actually the originally reported issue, the finalize_create_context_locked() part was something I found when the original report prompted me to audit gem_context_register() call paths. > > } > > > > int i915_gem_context_open(struct drm_i915_private *i915, > > @@ -2199,14 +2203,22 @@ finalize_create_context_locked(struct drm_i915_file_private *file_priv, > > if (IS_ERR(ctx)) > > return ctx; > > > > + /* > > + * One for the xarray and one for the caller. We need to grab > > + * the reference *prior* to making the ctx visble to userspace > > + * in gem_context_register(), as at any point after that > > + * userspace can try to race us with another thread destroying > > + * the context under our feet. > > + */ > > + i915_gem_context_get(ctx); > > + > > gem_context_register(ctx, file_priv, id); > > > > old = xa_erase(&file_priv->proto_context_xa, id); > > GEM_BUG_ON(old != pc); > > proto_context_close(file_priv->dev_priv, pc); > > > > - /* One for the xarray and one for the caller */ > > - return i915_gem_context_get(ctx); > > + return ctx; > > Otherwise userspace can look up a context which hasn't had it's reference count increased yep. I can add the Fixes: and Stable: tags while merging if no complaints. > > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx> Thanks BR, -R > > Regards, > > Tvrtko > > > } > > > > struct i915_gem_context *
